If you have too much water in there it can dilute the gelatin past the point where it will create a good gel. That could be one reason.

Another could be that you've gotten a lot better at removing the meat and have started to remove too much cartilage. You get a lot of gelatin out of cartilage and connective tissue that you may be missing out on when you remove the meat.

It is unlikely that the chicken is the culprit, although the small size may play a part (just not enough available relative to the amount of water) Even the worst CAFO bird will generate a thick gelatinous stock if cooked right. I used to make stock out of the .99c/lb roasters and it was plenty thick. So it must be some change in your process.

Throw in a couple feet if you can get them, they have a lot of gelatin in them. Also, chop up the bones and cartilage as much as you can. This is easier to do after they've heated up a bit. It's more work but I always get a tastier, more gelatinous result every time.

CAFO birds will also produce less gelatin, in my experience.

Gelatin is the result of the breakdown of collagen, the long fibrous protein that makes up a lot of bone, skin, and other connective tissues. When you heat collagen in water is breaks down into shorter fragments of protein, known as gelatin. It is these that form a gel when cooled.

If you keep heating it long enough these gelatin protein fragments continue to break down into very short fragments and amino acids, these will no longer form a gel however the liquid will have the same protein content.

Is ts possible that your crockpot has aged and the contents are being heated to a greater temperature than they were previously?

If you cooled the broth after 6-12 hours it might gel very well.

How are you determining that there is no gelatin? I can tell that there is gelatin in the stock when it cools, at room temp it will be slightly thick (depending on how much water), from the fridge it will be solid enough to rest a pebble on it without sinking (like Jello which is, after all, gelatin).

As another poster noted, if there is too much water, the stock may never "set" even if it has gelatin in it. I opt for making it too strong rather than too weak, so that it is easier to store and can always be diluted. I think you need less water than you might think. I usually barely cover the bones and use a pot that fits the bones rather snugly, so for say a whole chicken carcass, that wouldn't be more than about 1.5-2.0 quarts of water, less if you chop or smooch the bones together. I keep a lid crooked on the pot so the water doesn't evaporate.

Poultry meat doesn't have much of any collagen in it, it all comes from the bones and the "knuckles" and any connective tissue such as around the breast plate. So I wouldn't worry about cleaning too much meat off of the bones, but you should include the knuckles and all of the hard scraps.

Smaller birds (quails, pullets, etc) tend to have a higher "bone to meat" ratio and are often better for stock. The same is true if you can buy a collection of wings, necks, feet, heads, etc.

If you roast the birds and overcook them a lot, you might be melting a bunch of the collagen out of the bird before you make the stock.

I am not sure what temp most crock pots run at (I never use one), but when I make stock it is slowly simmering, so it is probably around 200-212F. If your crock pot has a very low setting at say 130-150F that won't be high enough to break down the collagen. That usually occurs at around 165F (which is why large bone-in roasts seem to "stick" at 165F during a slow roast).

That is about all I can think of. The most plausible explanation seems to be that you have too much water or too low a temp.

Reading through the above:

1. Are you sure you have no gelatin? Let it cool before judging.
2. I regularly make carcass broth. Don't care for eating chicken skin unless it's crispy fried, so I remove it and save to thrown in as well. I think this contributes.
3. Try breaking bones too.

My recent experiments have determined that pork shoulder bones yield an amazing amount of gelatin. When hot-to room temp, I thought I was in for disappointment and I only had two liter soda bottles to refrigerate in last time. Poured in and refrigerated. I had to squeeze the bottles to "bloop" out the "jello"!!!

do the birds have feet still attached?

Ok, leave the vinegar and livers out. Your missing out unmaking your own liver and onions anyway. Next, make sure to put the wing tips and feet into the stock. Add organic tomato paste. The acidity will assist breaking down the bones and marrow. Simmer below a boil for 24 hours. Strain the hell out of it.

5 lbs carrots 5 lbs onions 5 lbs celery .5 lb. Garlic 25 lbs chix bones, backs, necks 1 tbl. Black peppercorns .25 tsp. Crushed red pepper 10 bay leaves 1 cup tomato paste .5 oz. Wt. Thyme Rsemary if you want Parsley.... Water to cover top of everything in the pot Reduce after straining until gelatinous when cooled. Try cooling it. if it isn't thick enough, throw it back on the stove and continue.

I've been playing around with getting the broth to gel for a while now, and think I might have hacked it. My broth is now gelling so well it is now officially known in my house as "blub-blub." Here is what I've been doing:

Day 1: Place larger mammal bones, such as beef ball joints, neck bones, ham hocks, any leftover mammal bones into the crockpot. Cover with water, about twice the volume of your bones. Cook on low overnight.

(Note: I have seen the advice elsewhere on this site to let the bones soak without heat for an hour or two; I can't see any reason to do this, but have somehow done it accidentally the past several times--crockpot got unplugged--and so it may be a factor, somehow.)

Day 2: Add a good handful of chicken feet, plus any other bird bones you wish, along with your veggies and spices. I chop a whole bulb of garlic, and a good hunk of ginger, plus peppercorns and bay leaves. My local Asian market also sells whole fresh roots of turmeric, so I throw in some of those, and some shiitake mushrooms as well.

Cook for another 6 hours or so on low. Remove from heat, strain, refrigerate.

I have not been adding vinegar, or cooking my bones except when they're leftover from a meal.

The idea here is to get the maximum minerals from those big cow joints, and other heavy bones, while not overcooking the more delicate collagen in the chicken feet.

Like others have said, you may be using too much water - that is the only time my broth doesn't gel. The more I cook it and the more it concentrates, the better the result. I used to use chicken carcasses, but my experiments in getting the most gelatin into my broth has narrowed the bones down to feet and backs, which you can get at Whole Foods or most ethnic markets for very little $$. My broth looks like solid amber!