PROBLEMS WITH SDHC CARD

I have a Sandisk 16GB class 4 SD card in my Sansa Fuze.  It’s showing 567MG. available space, but won’t allow me to add any more files.  I’ve noticed that on the drop down menu under “View” that the “sort by name” selection has the “ascending” option greyed out.  when that option is not greyed out it will allow me to load more files.  Any ideas?