One of your tank vents is plugged. There should be a plastic tube that comes out of a fitting on the top and runs back behind the tank. Pull it out and clear it.
If you have shut down valves make sure they're both open. Gelling could be another reason. Had mine do the same last week. It returned fuel into both tanks but took it from one only. Hit the shut off valve a few times after I put 911 in and it leveled out.