No SXM Signal

Twice now I've tuned the radio to an XM channel and gotten the error message "No SXM Signal." I could understand this if I were in a garage or under a bridge, but this happened under a clear sky. I pushed the volume button to reboot the system, but this didn't seem to solve the problem. The first time it happened, after I turned the car off for a while and then restarted it, XM worked just fine. Has anyone else had this problem and, if so, is there a fix that any of you know of?