Potty Training Update

So...yesterday we did the potty party which was fun (we taught buzz lightyear how to go which Harrison loved), albeit tiring...Harrison put his big kid underpants on after a bit of drama and he can be convinced to sit on the potty (even for long periods of time...relatively..) but we can't seem to get him to actually GO in there. He doesn't have many accidents because I guess he just doesn't go that often and tends to drink most of his liquids towards the end of the day. We put him in a pull up for church and naps and he obviously gets wet (or poopy) and just doesn't care. We have a "special present" for him when he actually goes and he wants it because he asks about it, but still he's not making it happen. Any advice beyond wrangling him into the bathroom every 20 minutes????