Yes it can't be the Diaphragm Norm,Why it's not the diaphragm:If the diaphragm valve is open or closed the primer bulb should still return after being pushed , it must be a blockage on the pickup screen or blockage in the circuit to primer.
From memory that plastic fuel pickup tube in some of these has a check valve ball but most don't ,I usually remove the plastic tube under the carby and blow the tube and carby out with compressed air.
I used this drawing below to show the red fuel flow to primer on left side.
