Hi Lucian,
This could be an install state issue or package loading bug. We've observed situations where VS just decides it won't load packages for a session, and the NCrunch code never gets called. Try starting the IDE without loading a project, then opening your solution using the File menu (then repeat a couple of times, as this issue seems to be tied to a race condition).
If that doesn't fix it, I recommend trying the uninstall and reinstall fix documented
here (instructions are the same as they were for VS2017).