This is a rather hard thing to explain and I hope the image linked below will clear things up, but basically the arm of the character on the left should be rotated otherwise (the green dot should be at front) and I believe it breaks the animation I already made. Is there any way to fix this ?
This bug is often experienced by in-ROBLOX animators. So it is not only your game.
Another thing should be the parts that are welded to the avatar [left row of photos]. Those might or might not cause trouble if they are not all facing the correct way.
Other than this, nobody else can help you sadly.
Maybe you try to use the Worldspace for the Rotation or the Localspace. Or else you can try to make your Animation using Bones.