Almost always, this is lint. Over time the backing on a patch and the loop fabric on your pack pick up fuzz, hair, and dust, and that weakens the grip. Here is the fix:
1. Take the patch off.
2. Use a fine tooth comb or a stiff brush to gently clear the backing of the patch.
3. Do the same across the Loop Panel.
4. If either side got damp, let it dry fully.
5. Press the patch back on. The grip should return.