It's possible there is no material in the *.mtl file actually. Somethimes when files gets converted the material data gets stripped and all that is inside the *.mtl file is a default grayish material. Easy to check and that's drag it into Note Pad. Other reasons to is different directory structures and different file names. Like somebody edited the file names so they no longer match what they were when originally saved. Capitol letters, whatever lots of reasons.