Jabsco head on ericson 33

I have a Jabsco head on my Ericson ( manual pump ) . It’s been working well for 4 years but recently started to act up . It’s not able to suck seawater in when I pump, but if I put freshwater in the loop ( by turning on the sink ) , it pumps it into the bowl and out when I close the pump valve . Any ideas what the problem could be ? I checked the thru hull valve where water comes in and it’s open .