This seems very obvious, but it is one of those irritatingly obvious things that can slip past the radar as you can be convinced it is something else causing the problem. Hope this helps someone else.
I was frustrated because I had recurring calls to visit workstations which were not able to be remote controlled, even though ZENworks was installed on them.
I would reinstall ZENworks, and hey presto - remote control works. I'd walk away and then the next time the user would log a call, we'd find we couldn't remote control it again.
I found that if the PC was not logged in, it would be remote controllable; and if I logged in it was remote controllable; but if the user logged in, it wasn't.
After a lot of investigation, it turned out that "Use these settings and ignore user Remote Management Policy" was ticked. As soon as I took the tick out it worked fine. I'm not sure what underlying settings were enabled, as I find it
hard to believe someone would amend the settings. But it's something to
consider if you can't remote workstations in the future.