It can be that it's also close to its sale date, or a chartered cruise for a set group, or dry dock.
Depends what you are looking at and when.
Have you tried using different numbers in your group, if say it's not there on 4 but there on 2 it's filling up. If it's not there at all, is it near the cruise date? If it's just not in the normal pattern IE Eastern Western it could be dry dock.