If you've recently added a new mp3 into your iTunes (like buying from Amazon MP3), you may find that sometimes the file won't be eligible to be added to iCloud and you get that fun little cloud with the slash through it.
It may be due to the encoding. Try this to see if it helps:
1. Right-click on the song
2. Select "Create AAC Version"
3. This will create a duplicate in a different, theoretically compatible, encoding