hook and eye orbital sander problem
Assuming you're using the correct adhesive system for the pad, there are two instances where this will happen:

1) Your pad is old and will no longer hook the loop, you'll have to replace the pad, generally about 15$ and you can easily do it yourself.

2) Your pad is incompatible with the sander you're using(rare if you're using the same system). It happens sometimes even with the same system sanders where the hooks on the sanding pad are agressive as are the loops on the disc, causing the loops to tear off the disc damaging the hooks on the pad. This may have happened in the past.