I would confirm that the mix is fresh. If not- get it out of there and start fresh. Beyond that you are looking at carb cleaning. I could share some neat tricks if I knew what unit you have- maybe even a model #.

I've already seen where the filter wouldn't allow gravity feed until it was primed by some form of slight air pressure.

Before going to a lot of trouble--check the breather hole in the gas cap! Sounds like it might be plugged up with "mud", caused by a Mud Dauber Wasp laying an egg in the hole, then covering it with a bit of "poo"
and going away. Later, the poo hardens, the egg lies in the cap, until spring, the baby wasp arises, crawls out, and flies away!
In the mean time, somebody has complained about the engine on their lawn mower won't run more that 10 seconds, starts and dies! This also happens to any engine with an unsealed fuel system. Used to happen on cars and trucks, back in the "olden days"!

if there is an on/off valve on the ghas line make sure it is open. That's vertical on mine.