I can't remember what the inside looks like, but there's 2 tubes; The longer / taller one is "on", and the shorter one is "reserve". Usually, there is a screen to keep schmutz out of thos tubes. Perhaps your screen is covered, torn, missing.
You'll need to drain the gas and remove the petcock for inspection.
Years ago, draining and cleaning the gas tank was scheduled maintenance on BMW's. Depressing what collects in there...