When I want to reposition or recut a stamp set that I traded I put them in the microwave for 10-15 seconds (depending on the size- less time for smaller stamps). The rubber peels right off and the adhesive is nice and gooey - I've never had to add any additional adhesive before. The previous adhesive seems to melt a little bit and "freshens". Give it a try, you never know it might work.
PS. I've only tried this on SU stamps - don't know if that makes a difference or not.
HTH! Good Luck, Jennifer