Most Helpful Girl
You're not using the right size. Try a smaller one.
Most Helpful Guy
If she is wet, then it is most likely wrong size. If she is not wet, it could be the friction. Lube would help a lot with the second problem, but it would make the first problem worse.
A cock ring could possibly help keep it on, if you get one of the straight-forward kinds that don't have any metal. Any ring with metal will risk tearing the condom, so stay away from those. But you could find one that was silicon or something similar, that would help. Just.. obviously, put it on after the condom is on. Could help.