Help, Rig parts/bones not showing up in Roblox Animator

Hello, I use the Roblox Animator to animate my Viewmodels for my custom gun system.

I use skinned meshes for the viewmodel arms:

When I press the plus (+) button to add a new bone/part, it refuses to add the bone/part into the list.

Here’s the Viewmodel’s Children

This makes it really hard to animate, the animation still plays as normal in game when saved to Roblox but it’s horrible to animate. Please help.