I've only had my new C7 for three days, but am stumped with something that should be simple -- storing XM radio stations in memory. I saved three stations easily, but can't store any more. I tune into a station and press and hold memory button #4. It beeps and I release the button just like I did with the first three, but the station has not been saved. What am I doing wrong? (By "button," I mean the button icon at the bottom of the Infotainment screen.)