Initial steps:
1. Determine if the ISS is likely transmitting. Install a new battery in the ISS, turn on DIP switch 4, and verify that the LED indicator light on the SIM is flashing every 2.5 seconds. Then turn switch 4 off.
2. Take the console as near to the ISS as practical, and see if that helps.
3. Refer to Troubleshooting Wireless ISS Reception in your ISS Installation Manual (starts on Page 10 in most of those manuals).