MicroSDHC cards not recognized in Fuze+

I just picked up my Fuze+ (8GB) earlier this week to replace an 80GB mp3 player that died. I was excited about the ability to plug in MicroSD cards to expand the internal memory since the old player was about 80% full, most of it with video. I figured I could put the music on the internal memory and pop the video on the MicroSD cards and switch them out depending on what I was in the mood to watch. I bought one non-SanDisk 16GB MicroSD card at the same time as I bought the player, popped it in, and everything ran flawlessly (once I figured out what I was doing). Windows recognized it when it was plugged in, Windows Media Player recognized it when it was plugged in, I loaded two seasons of Doctor Who on, and life was good. I bought three 8GB SanDisk cards yesterday thinking I’d load more video on. I popped one out of the package today, switched out the 16GB one for one of the 8GB ones, plugged the Fuze+ into my computer, and…no external storage shows up. Not in Windows Explorer, not in Windows Media Player. I popped the card back out, stuck it in the adapter and stuck it into the computer, and Windows Explorer and Windows Media Player recognize it. Popped it back in the Fuze+ and still nothing. I tried it with one of the other cards to see if it was a problem with the card, and got the same response. And I don’t seem to be able to get the video to convert to something the player will see if I copy it directly over without having it in the player. What steps should I be taking to get the card to show up as external storage? Thanks in advance!