The problem can occur on either deck.
The METAL sensing switch is not being closed. The machine "thinks" it's recording on METAL tape, which requires a lower signal level. The culprit is a slightly small cassette case.
There is no indicator for the METAL mode.
There is no real "fix"; HOWEVER, If the customer is not going to use METAL tape we/they can short the sensor switch, at the circuit board, so that the machine "believes" it is always seeing NORMAL/CRO2 tape.

The connectors are easily accessable by removing the top cover.