I noticed that our org had some Accounts that had not been assigned to a territory, due to problems with the addresses. I corrected the problems, but the Accounts still did not get assigned.
I made the same changes in our partial copy sandbox and I saw that the Accounts were instantaneously assigned to a territory. The territory configuration is identical on sandbox and in production, so I don't understand why it's working in one place, but not the other.
There is a difference with respect to duplicate management: this is turned on, in production, but cannot be turned on in our sandbox. Does that affect territory management? I couldn't imagine.
The rules on when exactly Account assignment is run are subtle. For example, there is the option of making it optional on an Account page. But our org does not display that option on the Account page.