I downloaded a free font that included four .ttf files for the Regular, Italic, Bold, and Bold Italic variants, all different. I dragged them into Font Book and they all appeared in a group, but the three variants other than Regular were flagged as duplicates:
Multiple copies of this font are installed.
If I choose Resolve automatically all variants except Regular are shown as Off. If I right-click and enable these variants, ignoring the multiple-copies message, I'm still not able to use all the variants in TextEdit or Photoshop (the only apps I've checked in so far).
What's going on with this font? The previews in Font Book look exactly as I expect them to (rendering as the proper variants).
Best Answer
Have you, first of all, looked in /System/Library/Fonts, in /Library/Fonts, and in ~/Library/Fonts to see what actually exists ... as opposed to depending on software to tell you.
I expect that both Photoshop and TextEdit fonts behaving properly depends on the Mac default fonts - and fonts commonly used by those (and other) apps - being in the correct location.