Volume should be set at 3 o'clock. Student microphones are weak or cutting out. Student is too far from or too close to the microphone. Student is not in the 120 range of the microphone. Microphone is covered with something.
Cable is disconnectedcheck connector.
If another microphone works in same input, themicrophone cable may be damaged. The knob on the microphone mixer is tumed down too low. Remote site volume is too weak or too loud. Tape has record safety tab removed. Make sure you pushed the correct button on the VCR Record Switcher that corresponds to the site you want to record.