Hi,
On the problem track is the volume low or is there just no signal at all?
Check the signal path, or paths, from start to end in order and see where it falls down.
Is the tape recording stereo? If no, that's the reason.
You'd be better recording it to a mono track and only using the left channel output from your cassette deck/converter/etc.
No good?
Does the converter thing have two analog inputs, and the tape deck two analog outputs?
If so, swap them. Is your recording now exactly as it was before, or have the good and bad sides swapped round in your audacity?
If the former, only one side of your input path is working. (interface, audacity, computer).
If the latter, only one side of your output path is working. (Cassette/cable/player/converter).
If you track it to the computer side, Is your audacity track set to correct from the appropriate input numbers?
What make/model is this mp3 converter?
Hope something there is useful.