Maybe too much moisture or water. Dryer heat and sugar will help but I would try with dry heat first.
Just to add, I have heard of people using Splenda to substitute sugar. I have no idea what it would do to bark but it may be worth a shot in sauces.
Taylor-WSM 18.5 with Wifi Stoker, Kamado Smoker BBQ, and lots of charcoal. 180 club-Chicken