From my experience (I'm a classic user, but have used NG in testing), the upgrade files lag behind the ova files. And sometimes they don't even make the upgrade files. The most current upgrade file I see in Support is for 220.127.116.11, so still a bit behind the current ova 18.104.22.168. Do you have the auto-update feature enabled? I would think that would keep you up to date. You could try upgrading to 22.214.171.124 (if you are at a version below this). Then make sure auto-update is enabled and see if it upgrades to the current version.
You should contact support to see what they have to say about the availability of an update.