Diagnosis
The hook fabric on the pad is contaminated with dust and adhesive, or is worn out.
How to Fix It
Clean the pad aggressively. Use a stiff nylon brush or a dedicated hook-and-loop cleaning stick. Scrub in a circular motion to dig dust out of the hooks. If there is adhesive residue from old paper, use a small amount of mineral spirits on a rag to wipe the pad. Let it dry completely before use.
If That Didn't Work
The hook material is worn smooth. The entire backing pad assembly (DeWalt part N483707) needs replacement. It is not a serviceable fabric sheet.
Heads Up
Never store the sander with sandpaper attached. The adhesive and pressure during storage accelerate the wear on the hook material.