Check that the sanding pad you are using is in good condition.
Check that you are using the correct grit sandpaper
A course or rough sandpaper (80 grit or less) is better for removing larger amounts of material like stripping paint off surfaces.
A fine or smooth sandpaper (120 grit or higher) is better suited for sanding between coats of paint or varnish, or smoothing out material like rough-sawn timber.