If you are networked with another trainer or site you could have network connectivity issues. Use information from the DIS network Status page to determine if you are connected to other systems.
Provided you are connected to other trainers or sites check the following:
In MB 4.09 the radio objects may be set to permit reception of Plain+Encrypted transmitters or just Encrypted Only transmitters. Determine if your radio is set to Encrypted Only and the transmitting radio is in Plain mode.