It's either a clogged vent on the fuel tank or a clog blocking fuel from flowing from the pickup in the tank to the bulb. The clog cannot up downstream of the bulb.
Assuming it's not a clogged vent, one common cause of this is a blockage in the fuel line caused by de-lamination of the inner layer of fuel line itself. If the fuel line is more than 10 years old, consider replacing it.
It may also be caused by crud clogging the fuel pickup in the tank. Pull it out and have a look.
There may be a check (anti-siphon) valve where the fuel line comes out of the tank. Check that, might be stuck shut.
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