Here is a couple ideas.

1. Increase your baking temperature. 350 degrees seems really low. Most oven fries recipes call for a temp over 400 degrees.

2. Make sure the fries are all a uniform size. About 1/2 an inch wide strip are a good size for baking.

3. Make sure the potatoes are patted dry before you toss them in oil. I would toss them in oil vs lard. Better to have a thin coat of oil on your uniform pieces.

4. Spread them out over a cookie sheet. No touching! Each fry should have a little space around its borders. Clumping food together can actually cause the food to steam instead of roasting.

5. Lastly, if you salt, salt at the end of the cooking process. Right when you take them out of the oven.

Try those tips and see if they help. There are a couple other tricks that can help but they require a bit more work.

I use a non primal oxymoron microwave and first slice a hole in the yam or sweet potato--to prevent it from exploding and cook til soft. Then slice 1/4 inch thick while hot and pan fry in butter and coconut oil til golden brown on both sides. It is a great substitute for the hash brown potatos I used to consume in volume.

Sweet potato fries are tricky in that they have a higher moisture content. So you're not going to get the same effect as with regular potatoes.

One option would be to bake them at a low temperature to dry out, and then put them in again at a 350 degree temperature -- sort of like what I used to do with blanching regular fries: blanche them in hot oil at 275, then fry them again at 350.

Another thought is that they might be too close together/crowded. If they are, they're not going to get crisp on you at all. You might have to move them apart from each other, and break up your single batch onto two cookie sheets.

Don't know if you are opposed to frying them...but if not...here's my treat that I enjoy every now and then.

Julienne the sweet potato and fry them in small batches in coconut oil. Crispy and delicious!

I broil them rather than baking- that seems to work for me, plus its a lot quicker. Just watch them closely to make sure they don't burn!

Actually, the best I've made have been in the toaster oven. On high until they are soft and broil until crisp.

Secret to sweet potato fries?

Fry them. ;)

They're called FRIES for a reason.

I personally like a combo of bacon grease/butter/splash of EVOO. Sprinkle with mildly ground Himalayan rocksalt and dip in a home-made Garlic All-i-oli (Real Catalan Mayonnaise made with NO egg yolk :)

P.S. Did you know that Mayonnaise comes from Mahón on the Island of Menorca?

Yeah, I typically just make hash browns--shred in food processor, squeeze some water out, five minutes in a hot pan + flip + another five minutes. Way easier than oven fries.

grease the baking sheet with butter, cut them into uniform strips, drizzle melted butter on top, bake at 400 degrees for 30 minutes. No flipping or opening the oven. Remove with a metal spatula and season them afterwards.

cook them till they look done (yes they will probably still be soft), then place them on a wire rack to drain. As they cool on the rack they will miraculously get crunchy :)

I've had good results by coating them in some olive oil (suboptimal for cooking, I know) and a little coconut flour before baking; I can't remember what temp I used though, d'oh!

After years of either soggy or charred sweet potato fries, I stumbled upon a technique that has yet to fail. Like many above mentioned, uniform cut is important. Once cut, toss in a whisked mixture of egg white and olive or coconut oil. Let excess drip off and place on baking sheet with no fries touching. Bake for 20 - 30 minutes at 425F. Check often - SPFs have a tipping point that takes them from crispy to charcoal in the time it takes to pour another glass of red. Season with salt, pepper, whatever while still hot.