I'm making a new years card for my inlaws atm, and this post popped into my head! Whenever one of my stickers starts to peel i re-stick it with double sided tape. We bought a roll of the stuff for making these cards for about $5, but it's excellent for re-applying stickers; easy to apply and sticks real good. I'm not sure what glue you're using, but i'd imagine it's messier and way more finnicky than tape! Worth a shot if you have stickers that are otherwise fine but have just lost their adhesiveness.