Custom monster issue!

[xxxxxxxxxxxxxxxx]

If that helped, try "normalize weights", too. (Skin > Edit Smooth Skin > Normalize weights)It changes your skin weights so that the sum of all weights is 1 for every vertex. In 3D programs like Maya (and maybe Blender too?), it is sometimes possible to have more than 100% of combined joint influence on a single vertex, but game engines don't take too kindly to that sort of thing. Give it a try, sometimes while weightpainting you don't even notice the weights are actually not properly normalized!

(That's also what that normalize weights: Interactive option is for - It constantly normalizes the weights while you're weightpainting so that if you paint influence for one joint, the influence of other joints decreases for those vertices. Normalize weights: Post on the other hand doesn't do that right away so you can end up with weird values.)