It could be that the pad is worn out and needs to be replaced. Your pad has a number of tiny hooks on the bottom of it that grip the felt backing on the sandpaper and over time, these hooks deteriorate and the pad will need to be replaced.
To maximize the life of the pad, let the sander do the work for you. Applying pressure as you sand can deteriorate those hook much faster, but take away part of the orbital action of the sander as well. If it's been a long time, it is also a good idea to change the CFS disk (or brake) at the same time.
To order a replacement part, have your model number and type available and contact our Product Service Team at 800-678-7278, or order the parts online at ServiceNet, or search for a Factory Service Center near you. To locate the model and type of your sander, it is on the label on the side of your sander.