M.B.
My son, now 4 and uses the potty, did the same thing. The only thing I could think of that was the problem was that he would potty and the roll over on his belly and it would all "squeeze" out. It was very frustrating and I have no good advice really. I had one mom tell me she used to stuff cloth diapers down in the front of her sons diaper at night because she had the same problem. Might give that a try. Good luck!