I normally look for the following.
Lube everything and insure proper fitting. Then if that fails I carry a piece of spa flex where I by pass the filter to be sure it’s not a blockage in the filter. If that fails then I pressure test the skimmer line to see if it holds 20psi.
If there was a blockage in the filter, would that increase psi? If so, i can probably rule that out since my pressure has not increased, right? Would a blockage on the return side cause air in the pump?