You got a air lock! Fuel is leaving the filter faster than it can be delivered creating this air lock. Pushing down on the filter puts enough pressure in the line to release the lock. Use larger, longer and clear fuel line. larger line gives volume and serves as a reservoir. Longer line out of the tank keeps the filter from bouncing around. Secure the filter so it can't bounce around, that causes air bubbles in the fuel. Clear line allows you to observe the flow,
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