A cruise may not show up for several reasons.
Some TAs buy up blocks of cabins. Thus,
DCL will not show those cabins in their inventory, although they may be available for sale thru All Seasons Travel,
Dreams Unlimited,
AAA, or whoever. There seems to be some sort of system where DCL can ultimately recoup any of these that go unsold, but I don't know how/when that works.
Also, as stated above, as soon as you book a ressie for a child, DCL holds a space for that child in the appropriate club/lab group. When a group is full, they won't issue any more ressies for a child of that age. You can follow this by going to the DCL web site and making a ressie for 3 adults (it will show up if available.) Then try the same dates for 2 adults and whatever age child--if it doesn't show up as available, you know it is because that age group is full.
Same thing if you try to book a family of 4 and they only have 3 person cabins available.
If you are sure that you want that date and only that date, check several travel sites, and keep checking back on the DCL site. It is always possible that someone will have to cancel and will open up exactly the cabin you need.