What software are you recording into?
When using one mic and input, make sure whatever you are recording to is set as a "mono" or single track. If you have one mic plugged into the #1 input of the Mobile Pre, and you record to a "stereo" track, it will only be recorded to the left track. Input #2 would go to the right track.
The Mobile Pre Control Panel allows you to "direct monitor" the left input to both left and right outputs, but this doesn't route the left input to both tracks of a two channel recording platform. This may be where your confusion comes from... sounds correct when recording but playback only comes from the "left".