You might want to get checked for a MRSA infection. We have dealt with this on and off for the last few years. Wounds that don't heal are symptomatic. Bacteria love sugar, so your diet might be preventing a serious infection from taking off. What really works for MRSA skin infections (and we tried everything, including antibiotics--hibiclens cleanser worked for a time, but eventually lost effectiveness) is a topical silver solution. You can buy a really expensive colloidal silver spray, or just buy one of the new silver disinfectant cleaning solutions composed of water, silver ions, and citric acid (stings a little bit--citric acid is the "bait" for the bacteria). Spray the affected area about 6 times a day. In about three days, the boil, "spider bite", or cut that just won't heal, will start to heal up. CVS also makes a bandage with colloidal silver that works great for small cuts. (Alas, they are frequently sold out).
I've never had this problem before. but I agree with Kate that its probably systematic and you should get your white blood cell count checked (too see if too high), and get your platelet count checked (to see if too low).
try using honey and cover it with a bandaid or even coconut oil. both have cell repairing properties and speed the healing of wounds.
Vitamin C is a major indicator here, as you suspected, so is Vitamin A which is responsible for the formation of new tissue.
For C: I go for whole-source powders, like amalaki, camu camu and others rich in bioflavonoids. etc. It's made by Pure Synergy: Radiance C. Or go for Sodium Ascorbate... BUT take note: ascorbic acid is NOT real Vitamin C. Also make sure you're getting bio-available sources of folic acid.
Honey and coconut is also a good topical recommendation (see lalabomba above).
A possibility could be that you're actually doing too well at Paleo. Paleo is great at massively reducing systemic inflammation, however wound healing is actually dependent on a certain degree of inflammation being present in the body.
I wouldn't advocate that you intentionally try to provoke an inflammatory response, but perhaps try cutting back on the foods/supplements which strongly inhibit inflammation e.g. berries (particularly blueberries), fish oil, green/white tea, dark chocolate e.t.c.