Part doesn't appear in animation editor

I was testing out a way to rig gun tools when I encountered a bug. I can animate everything but any part inside a model doesn’t appear in the joint part list which is very annoying. It appears in the “add a new animation track” menu but clicking it doesn’t do anything.

here’s the clip: https://gyazo.com/40aa9b31c9ae91228162c5ae17b9dd0b

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