Hi,
Are you restoring from tape?
Are the drives currently in use by anything else, such as another backup or are they showing as IDLE?
If restoring from tape, can you confirm if all the media required for the restore is ONLINE?
To check this:
-Go to the Restore Screen
-Highlight the Saveset you selected for the restore
-To the right, under the Saveset/Index information, there is a "Media List" option
-Select that, and verify that ALL media is showing as ONLINE.