In 3.9, the "turn editing on" was removed from the "gear" menu and given its own button. But some themes took control of where the new button was placed, which is why there are problems (as far as I can tell.)
At first, I did what you suggest Rita. Yep, worked. However, if you have access (rights) to the Site Admin link, you could also click on Site Admin, and then (upper right side in Boost) click on the "Blocks edition on" button, then go back to the course. I found this to be a faster work around.
I believe Gareth is working on fixing the Adaptable theme. I do not use Adaptable, so I am unable to guide you much more.