fuel pump eccentric

Pull the pump and look for wear on the arm. If the eccentric wore the pump arm also would be wore.

I have seen the pump arm pivot pin on cheap pumps wear and causing your problem. They get loose and lose diaphragm stroke . Switch out the pump with a better brand.

Your problem could be other things

Gas cap not vented
Pin hole in fuel line or hose
Low float level
Bad return filter
Sock on pick up tube pushed to far onto the tube or tube without sock to close to the bottom of tank.

Rarely do you see the eccentric wore that far to effect pump operation without pump noticeable damage to the arm