First off, make sure that you close FH entirely after making a change to any material files, so that the game can reload all the materials and therefor have your new texture. If the material files was put into the my_objects folder, make sure that that's where you put the new texture. I'd also make sure that if you simply replaced the old texture, that you didn't make any spelling errors, or that if you changed the file type that you changed it in the material file.
tldr; Spell check, make sure you restarted FH, and everything's in the same place.