Could be many reasons.
Paint, Water or any kind of liquid in the barrel can make your paintballs do all kinds of wierd sh*t. Make sure its clean inside. If you have a break, use a fluffy or pull through. If its raining, you can put tape over the porting on the barrel (although this can raise the velocity apparently, and tape cant help if your diving).
Paint. If its bog standard white box, then it could be just that. Randomly different shaped balls, fragile balls, hard balls etc... Or you could be using old/leftover paint you have had lying around.
Paint to bore match could be a reason. You want the paintball to fit nice and stug in the barrel so it doesnt move, but still be able to put your mouth to the barrel and blow it out. So take the barrel off and put a ball in there and give it a go. If the balls are slipping through, you could get yourself a nice barrel kit that comes with various different back/bores to switch over to fit whatever paint your using on the day. Something like a RedZ Peppersticks set.
-Will