It depends upon why the mount has ceased to stick. Often, it is because over time, the edges have become roughened, and there is no longer a good seal, or because dust has accumulated on the disc.
Try cleaning the disc surface VERY carefully with window cleaner or alcohol, and blot any excess carefully with a soft cotton cloth. Do not rub with any abrasive material (not even paper towel). Moisten and rub gently.
If the disc still doesn't stick, a very small amount of water rubbed onto the surface with your finger before mounting may help.