I've had issues before with the air pressure blowing a hole right through the middle of the patch where the original hole was. I believe it's something to do with the chalk dust (or whatever it is) inside the tube blowing onto the outer surface of the tube just as I put the patch on, thus creating an unstuck cavity which the pressure can stretch out. I've found that using a slightly wider tube helps as it has to stretch less before it fills the inside of the tyre.

The best patches I've used are those made from an old tube as these are slightly thicker. However these need more careful preparation before use. This can be done at home though, and the prepped patches then taken on the bike.