monitor button inactive

Can anyone explain why I suddenly cannot use the monitor buttons on any Insrument track? They’re fine on Audio tracks, but on Inst. tracks they light up but don’t do anything, meaning I can’t overdub because I can hear the original take! I can’t seem to solve this or find it anywhere as subject (user manual, forums etc) and I didn’t knowingly make any changes anywhere for it to happen. Thank you if you can help.