Just as a devil's advocate: I agree that you need to rule out any medical concerns first and keep your patience. But, here is my story if all else fails. My son was fully potty trained between 3 and 3 1/2. Then, a year later, he started peeing his pants. He did this until part way into his Kindergarten year. That is frustrating! We did the whole rewards thing again. Didn't change anything. Eventually, I noticed that it was only at home that he peed his pants and most of the time had something to do with being denied something, like candy, playdates, etc. (although not always). He was controlling when and where and around whom he would pee his pants. Hmmmm.....so, I started punishing him for the behavior. Took things away, cancelled playdates, took away dessert, etc. It would only work for so long, then he would revert to his old ways. It slowed down drastically when I told him his punishment would be random (that way he couldn't plot and plan his punishment based on what exciting things were happening or what he was playing with). But, the thing that stopped it for good (he is now in 3rd grade) was threatening to tell his teacher. You see, he came home from school one day saying that he almost pee'd his pants when he had to walk home from the bus stop, so I should come pick him up in the car (our driveway is 1/4 mile). I told him he should have gone to the bathroom before he left school and that wouldn't be an issue. The next day, he peed his pants on the way home. I then wrote a note to his teacher explaining what happened and asked if she would allow him to go to the bathroom before getting on the bus. He came home the next day with dry pants and asked me if it was okay if he didn't give the teacher the note as long as he stayed dry.
Unfortunately, he had to decide to do it on his own.......nothing I could do would stop him it was completely under his control. I just had to help him find a reason to want to.