running out of room, reduced my music to 128 AAC but some files could not be converted?

[Tomb01]

So, have a huge amount of music, so decided to reduce the quality in my next synch. Went well and almost all of my thousands of songs succesfully got scaled back, but a few did not. Not sure I understand why, and the error message is pretty scant. Saw at least one place where someone said duplicates might cause it, but not sure that is the case in my situation. Any suggestions?