Could be a partial vapor lock problem, The bulb nor the pulse pump will move vapors very well. If the vst is full and the needle valve is close (not allowing fuel or vapor to pass) and there is vapor in the bulb or line going to the pump the bulb will not firm up because the vapor bubbles can not be purged due to the closed needle valve. When the temps of the fuel and vapor in the lines and bulb drop much of it can now return to liquid form which is solid under pressure which means the bulb will firm up, vapors are easily compressed and would not allow the bulb to get firm. Take following a steps to eliminate this possible cause
Top brand, top tier fuel
Bulb mounted arrow pointed up no more than a 45 degree angle, this allows for better purging of vapors and vapors will not get trapped in it
Shorten line if possible and be sure there are no low spots which can trap vapors
Try to keep the line going uphill all the way to the VST as much as possible
Use only a Mercury Bulb
No additional filters or water separator of any kind
Check the pick tube in the tank for any form of restriction
inspect or replace all fuel line, quite often they are restricted internally and this can cause the fuel to change to a vapor due to increased vacuum (low pressure)
If there is a check valve in the fuel pick up, be sure it is free and unobstructed, I do not feel they are needed in most cases since the bulb serves that function as well
Google
Vapor Lock and Reid Vapor Pressure for more on this subject