[nunit] NUnit Unit tests not showing in Test Explorer with Test Adapter installed

Just to add my $.02 here, I ran into a similar issue just yesterday, where 168 of my tests were missing. I tried most everything in this post - most especially making sure my version(s) of NUnit were the same - all to no avail. I then remembered that I had my tests divided into playlists; and these do not update automatically as you add new tests. So, when I deleted the playlists, BAM!, all of my tests were back once more.