Jabsco Water Pump loosing It’s Prime

Installed a back up DC fresh water pump but it keeps losing it’s prime. I installed a check valve (yes the correct way) in the supply line just after the tee for the AC and DC pumps. I have a pressure tank installed that was new last year set at 35psi. Is it possible the tank is back pressuring the pump? The DC pump will run and shut off normally but after a 1/2 day sit the pump will lose prime. I have to pull the output line off the pump until a solid water flow comes out then we are back to ops normal.