A long time ago, I had a problem with a 172 that had a faulty tank selector. With it set on both, the feed from one tank was shut off. Turned to either tank it worked fine. Your symptoms match that sort of scenario. I don't know what the engineer did to the valve in my case (replaced or stripped down) but it worked fine the next time I was in the aircraft. It could be a venting problem as well but you need to look at the fuel system diagram in the manual to see which is more likely. I can think of some ways to check it out on the ground but only looking at the manual will show what is the best for you. I would not fly the aircraft if I suspected a faulty fuel selector.