Could just be a cheap bulb. The valves in the bulb need to seal good to make the bulb go 'firm', but if they are leaking then it will always be soft. I bought a brand new bulb and line to use on a different tank and it does not get firm but it does prime the engine good enough for starting.
You just need to make sure you're not sucking air, because that could mean you also might not be feeding the engine enough fuel at WOT. Close the vent on the tank and squeeze the tank to check if air is leaking out anywhere. If it's not, you're probably good to go. Now, it might leak out the tank cap if it's got a gauge built into it, but that would not be a problem.