Let me guess: you are flying one of the new 172's.

Cessna uses a sending unit in these fuel gauges that is notorious for
breaking down. It flickers down to zero every now and then, also causing the
low fuel light on the annunciator panel to flicker. It is harmless, but it
gets worse over time and eventually the gauge will fail.

I know one 172 ME that has had its fuel gauges replaced half a dozen times.

Cessna needs to get on the ball and understand that this kind of quality
control crap went out of style when the Japanese hit the US auto market in
the 1960's. You would think that spending $180,000 on a new airplane
entitles you to a gas gauge that works.