Most of the time when this happens, it means you're actually wearing the armor piece that the transmog ornament came from. You can't transmog it onto itself.
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The arms I'm using are the Crystocrene Grips, I made sure to check this first just in case, but no luck
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There have been several mentions in past TWABS where specific armor ornaments didn't work. Hopefully this makes it on Bungie's known issues list.