Why would tube patch sometimes peel off with backing? Sometimes, when patching a tube, the newly placed patch will peel off when removing the backing. Sometimes I have this problem and sometime I don't and I'm not sure what the difference is. Is it the age of the glue/solvent? Not sanding enough? The temperature? The amount of time I'm waiting for the glue to dry?
I've found that the amount of time I let the glue dry makes the biggest difference. It really needs to get from "wet" to "tacky" to work best. Scraping the tube and the glue brand make a bit of difference too.

If the patch seems relatively well stuck, but comes up when you try to take the backing off, just leave the backing on; it won't hurt anything.