Man.

try applying or removing all scripts idk if anything would go through blender to source anyway.

Screenshots would probably help, but I would recommend using the newest version of BST on 4.1 since it's working fine with that for me. Other than that maybe you haven't specified a export path.

Make sure everything has an actual bone, in hlmv if the parts missing are inside the model at coordinate 0 than this should work.

I would enjoy a feature that removes/disables the flexcontroller part of .dmx when exporting if there is a way to do that already I would like help figuring it out.
With a lot of compilers using .dmx can add flexes to the list and reset most custom parts of it, this can get especially annoying when using multiple flexes for things like clothing following the base mesh on a customizable character.