That happened to me too sometimes, but here's the fix:
*Search for the model file (example: survivor_coach.mdl)
*Open it with notepad++
*Search for IDST0 (it's on the first line of text, so you can't miss it)
*Change the zero to one, or if it's already one, change it to zero instead.

That fix it for me.
Also, make sure the custom models and materials are inside a folder with a high priority in the gameinfo.txt

no its dont work because i try it and is crash

I copied the materials and models folders from the L4D2 mod and pasted it to the usermod folder/Workshop folder, or a custom folder if you have.

hey how you import this l4d2 costume models to sfm