People expect the bulb to stay hard once the motor is running. Not necessarily true. While you are squeezing it, it is filling the carburetors. Once the carburetor is full, there is nowhere for the gas to go, so it resists your squeezing. Now, start the motor and the fuel pump takes over and is sucking the fuel from the tank (and bulb). So it is under a suction condition, not a pressure condition as when you were squeezing it. Exception: If there is pressure in the tank, the bulb will be firm at all times.
Many primer bulbs have really crummy check valves. They work, but often you can't get a firm condition because the fuel backflows past the crummy valve, to the tank. Don't worry about it. If it fills the carburetor and you can start it, good enough. Also, gravity helps the check valves. Hold it so the flow is upward while squeezing.
Glass fuel filters normally run empty, or nearly so. That is to confuse the operator. Solution: Don't look.
