Yep, the ammunition companies schedule their production runs for various calibers, and that has to be fit between any requirements for non-commercial contract orders.
Also, as you might imagine, if it's a caliber that's not exactly in great and constant demand it's probably going to get dropped to a lower position in the normal productions schedules, too.
I've been told by a couple of ammo makers that they have to balance their commercial lines with their LE/Gov lines, and that also means they have to schedule their acquisition and use of the different components needed for their less costly (budget) line and premium lines when it comes to the inventory of the components needed for both.
Then, if they're planning their production runs a year or more in advance, and especially for a caliber that makes up a very small part of their commercial market, they may not produce enough to keep it on the shelves of some dealers, or in-stock with their usual distributors, until the next scheduled run (which might be a year away).
Even being a popular caliber that's in great demand may not help, though. I've known of scheduled orders for even highly popular calibers (like the Speer .38Spl 135gr +P GDHP SB) to require several months of waiting before some agency's order can be fulfilled.