ok, going to make an assumption that your .OBJ and .MTL files reside in the same folder.
That being the case, can you open your .OBJ file with wordpad (or similar text editing app) and see where the MTLLIB is pointing to?
In example below the MTTLIB is pointing to Coffee_Pot.mtl, and because there is no path (for example 'C:\temp' or 'D:\CoffeePot') then the OBJ file assumes it doesn't need to look outside it's own directory for it.
If there is a path name qualifier, then delete it (or comment out), leaving only the MTL file name, then try to reload your OBJ file and see what happens.