When you are recording the meters are probably reading the input signal not the tape playback. You can switch them to monitor the actual tape playback while recording. The problem may be the playback if the meters are not reading both channels. As to the problem it could be many things especially on a machine of this age. Dirty switches and bad relay contacts are two of many possible problems. The heads could be bad. The record or playback electronics could need service. You will need to get it to a good technician to give it a once over. Could be expensive but a complete rebuild would not be a bad investment if you plan to use this deck long term.