Missing vertex groups from Transformers: Earth Wars and other games

Hi, I've been ripping models from Transformers: Earth Wars for years, along with their textures and animations. A very small percentage of those characters are missing their vertex groups when extracted. The mesh, textures, and rigging are all there, and the animation is animating the rigging, but there is no vertex group/weight painting information included, so the mesh doesn't know which parts should move with which bones. The process I use to extract is the same as what it is for models that DO work correctly, but the vertex groups seems to be missing. I don't understand it, because the models move and animate correctly in the in-game galleries.

This is becoming a bigger problem as time goes by, and I've seen it start to affect other games as well. I'm using AssetStudio, if that helps.  Any ideas?