Pokémon Model Ripping Project

[Yunpol]

Can anyone explain to me why Pokemon that have jaw bones tend to have their weighting messed up upon importing the model? Like I know it's fixable, but It's really annoying to deal with.

http://i.imgur.com/mKom1w4.png
http://i.imgur.com/zbvDIs1.png

Sometimes this problem extend to the hands, feet, and feeler bones as well. (e.g: Emolga), which I don't know why that occurs with certain Pokemon.

Going to say this again since it was never answered.