DVR won't record while we're watching something else

I am aware that something can record on my DVR while we watch something else on another channel, or at least that's always been the case. I am also aware that if two things are set to record at the same time, only one can record if we wish to watch something else, or we can watch one of the programs that's recording while the other one records at the same time.

But in the last couple of weeks, our DVR has decided that it can only record one thing, and we have to be watching that channel while it does so. We can't change the channel during a record or else it asks if we'd like to cancel the recording to change the channel. There are no other conflicting program records that should be causing this and I don't know what to do!

This just started happening and we've never had this problem before.