The transponder isn’t replying to interrogation or commands

An incorrect transponder address is set in the Topside Control – check the address and repeat using the correct address.

Operating at long range – increase transmit power and receive gain to the surface transceiver (Ranger 2).

Surface thermocline refracting reply signals – lower transducer below thermocline.

Noise level at surface transceiver too high – lower the dunking transducer – reduce the surface transceiver gain settings (Ranger 2).

Transponder battery power low – check battery levels.

Other acoustic systems maybe operating in the same frequency band.