monitoring in Resolve can be a bit of a wormhole, so without screenshots of several other areas of the program, it is going to be hard to pin down the trouble.
Some things to check.
Are you clips panned left and right
Are you hearing sound out of both speakers
are you set for stereo monitoring, mono monitoring, 5.1 monitoring
do you have audio busses set for something specific?
If you drop audio on Ch1 down to 2, can you hear it?
If you raise 2 up to 1, can you hear it?
Are you sure both audio sources are still available to the program.
You said at the start of your post “suddenly” does this mean at some point you could hear both tracks as they are currently laid out? If so, what did you change.