I'm trying to record vocals and I simply cannot figure out why I can't hear the vocals as I track through Reaper. I've done this a million times in the past but just can't see what I'm doing wrong....

So I can hear the backing track and I can bounce that around wherever I want via the interface UI, but I can't get the vocals coming through the headphones - now they ARE being recorded, no problems, I just can't monitor them as I record. And yes, I've tried all three positions of the track monitoring button - all the same. Nada.
I got around it temporarily by plugging the headphones into the interface and doing some re-routing there... so I'm not sure if this is an interface thing (strong suspicion) or a Reaper thing, but I haven't changed anything that I'm aware of, since I last recorded vocals.
Any suggestions?