Here is some advise from Tinkerdave, one of our most active forum members on this subject.
Velcro on your sanding disc has a hard time sticking due to dirt ,saw dust and other debris getting stuck inside, which in turn makes it hard for the hook and loop sides to stick together. Because of the high torque loads placed on the hooks and loops they become distorted and damaged.
Before you throw your pad away first try this:
Remove the velcro pad off the sander and lay it on a flat surface. Take another piece of Velcro and comb through both the hook and loop sides, allowing the dirt to lift. Place another piece of Velcro on top of the older one hook on top of loop or vice versa and press and release it several times. This will loosen the dirt and take the majority of it link removed a metal toothpick small piece of wire to pick through the rest of the Velcro.Blow it off well with air. By doing this most of the dirt and saw dust should be gone. The toothpick or wire will help you get the rest of the dirt out.Hope this helps.Good luck.