I agree, 2 months is a short amount of time. When we started behavior therapy, our therapist told us at the beginning to expect the behaviors to get worse before they got better. As he tested his new boundaries to see if I would stick with it, or give in and give him the reward without the good behavior. My son placed value on any sort of attention (both negative and positive) so it was months, upon months of ignoring negative behaviors and rewarding even the slightest positive ones.
One thing we found really helpful was if my son got to pick the rewards. He got to pick what reward he felt was appropriate for what good behavior. We incorporated some of these rewards into his school day. Since he spent some of his day in the Resource Room (Special Education Room), if he modeled appropriate behavior during the school day he got extra computer time or some lego time in the Resource Room.
It took MONTHS, and in the beginning I thought it was pretty dang stupid. But gradually we saw improvements and we started increasing what was expected to obtain an reward. Hang in there.