Since you say you have set up (1) stereo output bus that has both L & R audio interface outputs assigned to it, I have two another thoughts/suggestions for you to check.
1st… An issue could happen if you recorded a mono source (like vocals from (1) microphone, or a guitar from (1) mono connection) from a stereo input bus by mistake. These mono inputs should have been recorded through a mono input bus (not stereo). I am not sure if you can fix this except for trying to adjust the panning in such a way that both sides sound close to each other, then using the track volume to raise the volume to an acceptable level.
2nd… An issue could happen if you recorded your mono source vocal from a mono input bus (as you are supposed to do) to a mono audio track but then you assign a stereo VST effect to that track (like a reverb or some other kind of phasing effect). If you do this it is possible that only the left channel of the reverb will be heard (that’s the way Cubase works and it is documented in the operation manual). If this is the case then you may be able to correct it by adding a stereo audio track to your project and copy your mono recording to it. Then add the stereo VST effect.
For the 2nd issue… there are several ways to get around this issue but, for me, I always record my mono input bus for mono vocal or instrument connections to a stereo audio track. Then you don’t get the issue I mention above.
These things are confusing for sure. I hope you can understand by reading my explanations carefully. If not, just ask more questions and I will try to assist.