Not all songs are picked up by the player

Hi, everyone.  My first post.

I bought a clip + a few months ago and love, but noticed something that has shaken my confidence in it.

I added 60-odd songs to the player in the last hour in a folder, which has 3 subfolders, which have the songs in them.  I can connect the player to my computer and see the songs.  

However, after the player regenerated the database, only about half the songs I added this morning are showing up.  I dug down to the folders level and cannot see one of the subfolders at all; a second one only has 13 of the 23 songs in it (even though, like I just stated, I can see every one of those tracks when I hook the player to my computer).

The only clues I can offer you are that 1) This folder is on the micro sd card; and B) The 16gb card now only has about 2 megs of space left on it. 

Am I filling the card up too much?  Do I need to leave some “breathing space” on it so that it can fully generate the database after an add?  Do I need to check the other folders I added recently to see if all those songs are showing up?  (I have noticed that some artists seemingly never show up when I’m in shuffle and no repeat modes).

Thanks very much, everyone.  I’ll check this forum later on today.