I just rebuilt an s and s and had the same symptom. I turned out I had reversed the spring and diaphragm. I can’t remember now but I had put the spring over the diaphragm rather than under it. Or maybe visa versa.
Could that be possible in your case?
that could be a possibility carb is not very old and not alot of miles on it cant see worn parts being the issue don't think the pump worked from day one will pull it apart and check your suggestion but leaning towards a weak spring a this point