First off, you're using the good patches with vulcanizing fluid, like Rema, right?

Make sure you rough up the tube and let the glue completely dry before applying the patch for the best bond. Also, if you remove the clear plastic, fold the patch a couple times to make the plastic tear then remove it from the inside rather than risking pulling up the edges of the patch.

I've had to rip off and re-do a leaky patch before, mainly because I didn't follow my own tips above.