When a Song's recording mode is set to MAS (Mastering), only tracks 1-4 are usable and tracks 5-8 cannot be used.
When creating a new Song, be sure to select MT1, MT2 or LIV as the recording mode in order to use all tracks 1-8. Additional information can be found in the "Creating a new song" section on page 42 of the Owner's Manual (note that this is page 80 in the PDF file).