Why can't I import Garage Band clips of songs into iTunes?

Using a MacBook Pro, I recently began a project in which I was using Garage Band to cut songs into short 20–30 second clips for a church activity. I simply started a project using the loops setup, dragged in my music, edited them into small clips, imported them into iTunes, turned them into MP3s, renamed them, and finally put them in a playlist to be used later. It was going well for the first two songs, but I’ve run into trouble importing a third edited clip into iTunes. It was saved just the same way the others were, and the project and Garage Band were closed. When I selected File > Add To Library and selected the file, there seemed to be no response from the computer. When I tried File > Library > Import Playlist and selected the same file, a dialog box appears saying “An unknown error occurred. (-50)” How do I import this song into iTunes successfully?