The check valves can get sticky after sitting for a long time. I free mine up by squirting a little fuel into the inlet nipple using a fuel bulb. You don't need much, maybe a teaspoon full (approx ) and then connect the inlet and outlet nipples with a short loop of fuel line.
Let it soak for a while...to loosen things up...
You might need to do it a few times...
You can get an idea of the pumps function by flipping the prop and watching to see if some fuel moves through the fuel line loop from outlet back to inlet.

Check for leaks from the vent/weep hole...and you can also remove the backplate and look for any fuel comming through the 4 little pin holes.