If the co2 tank isn't empty, , then maybe the co2 is not entering keg or maybe there is a small leak affecting carbonation just enough for it to matter but not enough to drain the tank noticeably.
Have you shot a solution on all the parts that can leak and tested to make sure co2 is flowing and can get into the solution?
What about a bad co2 gauge reading? This has happened to me. Thougt it was 30 psi because thats what it was reading but turns out it was inaccurate and it was only 20psi