Some of the stickers on a new 5x5x5 from Meffert's were slipping. It looked like the stickers were just missing glue altogether. I tried a liquid glue called "Plastic Weld" that's used to bond styrene, since I had that handy. (Available at most hobby shops.)
The good news is that it worked just fine. The bad news is that the sticker and the cubie are now essentially a single piece of plastic and the sticker probably can't be removed without destroying it or the cubie. That may or may not be a problem from your point of view.
If you try this, be EXTREMELY careful and use only the smallest amount of the plastic-weld stuff, since it effectively melts the plastic and too much would liquify the sticker. Also, give it 24 hours to dry.
And don't accidentally get any into the cube unless you're intentionally trying to bandage it - the stuff works great for that.