I have something very odd going on. For most of my fonts the preview works fine, but for one of them, it works fine if I select particular weights, or any weight, but if I select show all instances, nothing shows up.
Just gone back to the last non-beta release and all fine. Odd that it happens on only one font and not the others. To that end. I have just gone back and tagged this thread with the bug tag. The version I had been using was the newest 3504.