That is a puzzler, especially since we have had the opposite experience when booking that far out.
Maybe because the holiday sailings are more popular and therefore book much faster than others?
We did have an experience a couple of years ago, where we booked a sailing ~1 year out, and our confirmation email indicated 'wait listed' for early fixed dining.
This prompted a quick phone call to Ship Services, and the rep promptly assigned us confirmed early seating. Took less than two minutes on the phone.
So even if you get an initial wait list assignment for your preferred time, don't stop there; call them and they should be able to work it out, especially that far out.