Other users in the same UserGroups DO see the button.
The reporting user DOES see the button in other nodes (presumably with the same platform, browser, etc).
Permissions Analysis in each case for the User vs. the Node show "Post New Thread" = yes.
Both forums affected have "Threads limit" = 0
Users have been advised to clear cookies, cache, log out and in etc., and apparently the problem persists.
Anybody got any clues or suggestions as to further analysis we can do to throw any light?