It's a UI thing.
if you're wearing the piece of armor that you're trying to unlock the tranmog from, it won't allow you to view it. Solution -- put on a different arms piece, then go into your transmog menu and you should see it.
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I have tried that already. I learned that with my titan
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Okay. If you've tried that and it's still not showing, I would recommend uploaded screenshots (if possible) to a host site like Imgur.com. I'll move this over to the help forum. Most of the team is off right now until after the new year, but others on the forum can offer additional suggestions or if there truly is a bug for those specific arms, Bungie may have to look at it themselves.