This happens when the clear top coating on a decal sheet goes bad for some reason. You can recoat the decal with one of the following: Microscale liquid decal film (brush), a crystal clear acrylic coating (spray can - Kryon is one brand), or Future floor wax (brush). You want the coating to be neither too thin or too thick. One coat of the liquid is usually enough; a few thin coats of a spray works for me.
Don't be too exacting with the area of the coating - spray the whole decal sheet if you're using a spray can. After at least an hour, cut the decal you're going to apply out of the sheet, leaving as much area around each decal as you can. Scissors are best for this.
Then, with a new x-acto blade, cut close to the decal image, but not all the way through the sheet - it's a lot easier if you have some paper to hold on to. Dip the decal in warm water, and let it sit in a little puddle of water. Test the decal by trying to move the coating that's outside the cut you've made around the decal; once you can move it, make sure you remove all of unnecessary coating from the sheet so it doesn't get on your model. Don't get impatient about moving the decal itself - test it gently with a paint brush or a cotton swab and wait for it to release itself and be easy to move. You can add more water to it with an eye dropper or even redip it if it's stubborn.
Good luck.