Hi Lee !
Open the .mtl in a text editor and check all texture paths (use 'notepad++' to search & replace over many files if needed).
Usually if you want the .mtl right next to the .obj file, use a 'relative path', means just use the texture name.
E.g.
If the path in the .mtl is :
mapKd C:/images/myPic.jpg
CE cannot find that path because it needs to reside within CityEngine's workspace. If the files are copied to :
assets/..
it should work with just the texture name.
* * *
Otherwise, you can also use the standard file navigation syntax :
../ # 1 dir up
/ # 1 dir below
* * *
I usually create:
assets/trees/..
assets/cars/..
..
and within those just use relative paths in the .mtls.
Additionally, you'll make your life easier if none of any textures you use in your full projects share the same name because on export, they may ( depending on your export settings ) be copied into 1 big folder. Note that files will not be overridden if they have the same name, but incrementally renamed, which may mess up your workflow.
Cheers !
Matt