The 'low signal' on one channel of a cassette may be due to a mono recording. The gap between the tracks (one each way) was bigger on mono than stereo so a mono recording played on a stereo player would by nature have one strong channel and one weak. The weak channel is likely to suffer far more from tape noise.

When I have a mono cassette to transfer - in Cool Edit - I transfer in stereo then delete the weak track. I select the strong one and paste it over the deleted weak track.

However, it is worth checking which is the better track - I have had a few instances where the strong on has more obvious distortion. In that case I'll use the weaker track and attempt to remove some of the hiss.