I'd guess a serious lack in adhesive!
Micro-set has given me issues in the past...so maybe there's "something" there???
I have completely given up on that stuff. I replaced it with "Future"...which I use in the same way that you would use micro-set...under the decal. If you have Future on hand, you could use that to put the decal back in place...if not, mix a little bit of white glue (or Testors clear parts glue) with a little water...brush it where the decal is to go, place decal, and dab out the excess.