The problem is that the military build screen has a couple of places to the right side that
look like empty slots for unit cards, which suggests (at least to me, and apparently to others) that you are currently displaying all the units you can build. Those apparently blank slots are misleading; always try scrolling to the right, because there's a good chance you have more potential units.
That's not a very clear description, a screenshot might help:
The area I have outlined in red, in the lower right-hand corner, is what I'm referring to. It seems like there's enough room for another unit card there, so since it's empty I assumed I could only build five types of squadrons. But in fact there are several more that I can reveal by pressing the right scroll button.
EDIT: This problem shows up in all unit categories, not just ships. It's even more misleading for land units, because the extra space is precisely the right size for one more unit card. I'm embarrassed to admit how long it took me to realize the problem.