This sounds like there is a bad connection in the signal path somewhere…possibly a short or an open somewhere. Judging from the fact that reflowing some solder joints fixed the testpoint readings, it seems likely that you probably have some other bad solder joints somewhere. I’d very carefully re-flow all the joints on the board, particularly in the signal path. Take your time with this because pads can easily get bridged.
Good luck.
-Brach