fuel gauge issue -'68 Barracuda

filled it up a couple days ago (which I probably have not done in a year or so). Anyway it seemed to stay at full mark for too long after I drove about half a tank so I tapped the ralleye dash bezel a little bit and needle began to move downward but it kept going to the empty mark. Tank has to be at least half full. Any ideas what may have caused this? I turned ign. on and off a couple times to see if it would start working but no dice. I think temp gauge is still working OK. Ammeter has never done much. Not sure what to check first. thnx for any tips.