Can you detect any air leaking around the squeeze bulb fittings ?
How old is your squeeze bulb ? How about any cracks in the rubber bulb.
There is a separate rubber bladder inside the bulb that contains the gas.
Then, there is the outside part, just like a football.
Then, there are two hard plastic balls that allow the fuel to flow only in one direction.
They are not spring loaded, they float freely. So the bulb must be at an angle or
straight up and down in the right direction for it to operate properly.
A combination of the fuel line fitting at the motor and a "less than good" squeeze bulb
could cause this fuel delivery issue.
If you have already eliminated the fuel pump, carburetor and vent on the gas tank, the next
area to check would be the fuel line connectors, hoses and bulb and clamps.
Pump up a lot of pressure while wiggling and jiggling the fittings and where the hose clamps are.
They don't last forever.
I myself, and many others here, have suffered the same dilemma that you describe.
If you can install your filter and squeeze bulb in a vertical position
you may have at least improved the fuel flow.
On my 25hp Johnson sitting on a 15" transom, there is not much room for the vertical installation,
there is only about 3" between the filter and squeeze bulb, but it works fine.
oh, :WELCOME: aboard !!
once you are confident that you have fixed your problem,
next time, you can share your experience when someone
asks about the very same problem.