While I cannot tell you why some buttons are unusable (greyed-out) on some workshop scenarios, I can certainly say that it happens frequently. Unsubscribing and resubscribing does not usually help. If you know which route the scenario runs on (which is not always easy to determine), bring up the Drive tab; then select the scenario's type tab (most likely Standard for Workshop scenarios, but not always). Find the scenario's name in the list and . . . take it from there.
A few days ago, I found an easier way to do this, but I'm not sure that it will work in all cases. Follow the first two steps above, then enter the scenario's name into the search box and press Enter.
EDIT: now that I've thought about this, I may have been able to simply get to the Drive tab and enter the scenario's name in that tab's search box. It's worth a try?
