INSTEON: Troubleshooting Communications Errors
From Universal Devices, Inc. Wiki
Communication Problems with Some Devices
Multiple users that have had similar problems have found that 'stacking' one of their SignaLincs in the same outlet as the PLM helps to resolve this. One user has also reported the same success with the newer Access Point. *Be sure to 're-link' both PowerLincs after moving one to ensure that both are communicating properly AND on opposite phases of your power line.