Skinned Mesh Rig has transparent areas where parts connect

I’m currently working on a S15 rig for personal use and I’ve hit a roadblock which I’m unsure how to fix and haven’t found any documentation on it yet.

In short, certain parts of the mesh (Where there is weight) just doesn’t load at all In Game as I said in the title

Blender Version:

Running on an actual Player:

What’s Stumped me is why this only happens on the RightUpperArm, UpperTorso and UpperLeftLeg. If you need any more info at all to help let me know!