Character skinning.

I remember that last time I tried to do something with customizable character, I came across this problem..

Say I have a rigged character model and done some animations for it as well, it has a mesh, skeleton, is skinned and works just fine with Unity.
But if I try to add another mesh to it, or modify that current mesh, I have to use Maya or other tool to re-skin (attach the mesh to bones) again so that new/ modified mesh would stick to skeleton.
How ever, that re-skinning makes it so the animations I had for the same model before re-skinning doesn't work no more in unity as if it was a totally different.. I don't change, add/remove or rename any bones, only modify the mesh and re-skin it in maya (reattach mesh/skin to skeleton) so why doesn't it work?!

Am I doing something wrong or what? I wouldn't call myself an expert when it comes to this, so could someone please kindly explain what I'm doing wrong or what the problem is, because I really need it

Thanks

EDIT:
If you don't understand what i mean, please say and I'll explain again

 

aw bump!

 

Erm... perhaps you could expand on what you've said a little?