New Songs Won't Play On SD Card?

Personally, I would just start from scratch, again:  move your content from your player and SD card to your computer; reformat the player under its System settngs; reformat the microSD card (you can use the SD Association’s formatter tool for that); re-put the latest firmware on the player (see the firmware upgrade sticky thread at the top of the forum); and move your content back on your player.  Hopefully, this combo. platter refresh will help.