My Sansa Clip+ 4 GB used to record FM radio all right - now it doesn’t. I am all despair - I’ve checked in both modes (MSC and MTP) - no effect. I hope for some help on the forum.
You might try manually reapplying the latest firmware, in case this is being caused by a software glitch and you haven’t yet tried this. Easy and quick to do–see the firmware upgrade sticky thread at the top of the forum.
And/or, possibly, a reformat of the player under its Settings, although I’m not sure that this would do anything here (altho, it can’t hurt, if the above doesn’t work). Note: a reformat erases the user content on your player–transfer the content to your computer first.