Best place for posting this question would have been in the section specific to your operating system and the Audacity version that you are using (for example Audacity 1.2.6 Windows).
So I assume that everything is working but you have one specific recording that is a stereo recording, but only has a signal on the left channel and the right channel is silent?
Open the file with Audacity
Click on the name of the track in the track panel and select “split stereo track” from the drop down menu.
Select all of the left channel and go to “Edit → Copy”
Select the right channel and go to “Edit → Paste”
Now select from the file menu “Export” (either wav for full audio quality or a compressed format such as Ogg or MP3 for a smaller file) - the tracks will be automatically recombined in the exported audio.
Or are you only getting a sigle mono track when you record?
If so, then your problem is that when Audacity is fired up for the first time it is set to record in mono. You will need to go to Edit>Preferences>Audio I/O - and set Channels to 2 (Stereo) from the dropdown. Therefter Audacity will remeber this setting for you.