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.