I tried to put the mtl and obj, and any separate jpg, png, etc. in a zip all together. Sometimes that works, most times not.

Please note that the MTL file is referenced from the OBJ file with an address. If, when combining OBJ and MTL in a ZIP file, the (relative) address changes, the OBJ file cannot find the MTL file, and will ignore the date contained in it, resulting in an object without colours and/or textures.

When this happens, check your OBJ file to know the exact address, then either change the address in the OBJ file, or make sure that the MTL file is where the OBJ says it should be.

H