Look for some strings to pull at with a few machines that regularly check-in but never update inventory.
If we re-enroll them they'll update inventory on enrollment but never automatically after. Occasionally we can run sudo jamf recon but eventually even that seems to fail.
jamf.log indicates an "unknown error" over and over and over
Tue Aug 30 20:18:22 LAPTOP jamf: Checking for policies triggered by "recurring check-in" for user "USER"... Tue Aug 30 20:18:26 LAPTOP jamf: There was an error. Unknown Error - An unknown error has occurred.
In searching the forums here I've seen a few times to disable "Collect available software updates" during inventory collection (Settings > Computer Management > Inventory Collection) due to a character length issue with some Apple updates, but alas that doesn't seem to have helped, nor did the above error seem to indicate it was an update concern.
I know this issue can be a number of things so I'm just hoping for any ideas I can dive into on my side as to what it may be!
We remove and re-enroll machines in our environment. In our case, it turns out there were duplicate records for the hostname in the database. If you reuse machines - removing and enrolling again - consider this and reach out to Jamf Support. They provided a MySQL command to reveal the db had duplicate entries for the hostname.
Although as you said - it can be multiple things - depending on the specifics of your environment.