as all others have suggested, you need to WET SAND when sanding down topcoat paint with anything coarser than, say 180 or so. primers can usually be sanded dry to much finer grits.
when dry sanding, once the paper starts to clog, additional clogging is accelerated - the paint likes to stick to itself.
i always use a bucket to hold the water. a little dish soap in the bucket water can also help if you still have clogging issues while wet sanding. but really, if the paint easily clogs on the paper while wet sanding, it is not cured properly.