It's pretty well established they build in batches of Caliber/length/configuration. If you happen to order while a certain configuration is being filled you have a good chance of getting it quickly. If they aren't working on a particular batch... Well it's a crap shoot as to when it'll get to you.
For example:
6.5G 18" stealth Black ordered 12/9 shipped 3/24
5.56 16" predatar Black ordered 12/12 shipped 2/12
6.5C 18" stealth Black ordered 1/12 still waiting
.308 18" predatar Black ordered 1/20 still waiting
So if you see a trend of a configuration you want in the UU wait times thread, jump on it. If you're early enough you might get in on the batch. It seems like a few people have fallen through the cracks though. If you see your configuration going out from a similar ordering date and you get passed over, I would call. Mistakes happen. Especially when they are working at capacity. Larue is churning out UU as fast as they can because of all the panic buying and the riots creating so many new gun owners. They can only build them so fast.
As far as buying new machines, well that's not as simple as it sounds. These machines can be 500k+ dollars. Not to mention the new infrastructure to house and run them. You don't buy machines to keep up with a panic surge, you get machines to keep up with normal demands. If you try to buy enough machines to keep up with panics, you don't make your money back on them, and are left holding the bill for millions of dollars worth of equipment and infrastructure that is just sitting there taking up space because there isn't enough demand for them to be paying for themselves in normal times. It's the same with ammo manufacturers. I bought my first UU on 10/5/18, and it shipped the same day. Thats normal demand, and I bet they still had some machines idle and not paying for themselves. Ammo was plentiful and cheap at that time as well. We would have a lot less manufacturers, both guns and ammo, if they went out of business because the made poor business decisions like buying machines for panic demand.