Can't Add to Library

I imported 6,300 songs to iTunes from an external hard drive yesterday. File formats were mixed between MP3 and WMA files, but iTunes seems to have converted all the WMA files to MP3 w/o a problem. Now, however, when I try to add from a different folder on the same external drive, nothing happens. All files are WMA files that I recorded from vinyl, so they are not protected files, obviously. I went ahead and dragged and dropped the entire folder of songs to my local drive and then tried importing locally, but still nothing. Tried dragging and dropping from folder to iTunes, but again, nothing. File format shouldn't matter, since everything I've read tells me that files should convert to the specs I choose upon import. Any ideas?