Skeletal mesh broken into random fractions in LOD0 when import into UE4

Hi , I ran into this problem today and have been trying to find or debug the problem for the whole day but still got no luck so far.
I have a skinned character consist of several different meshes, all are applied using a single simple lambert material., when I bring her into UE4 as skeletal mesh, UE4 broke my skeletal mesh into 2 or few more different seperated sections in the LOD section even tho I only use one material for the whole mesh. I attach an image to show the issue.
I can’t use the skeletal mesh like this as it would ran into problem with some shaders I created if my mesh vertex is not merged
any help or suggestion would be hugely appreciated :(( :(( !

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