Have you cleaned the track recently? Also clean the wheels on the dump car. It could easily be a dirt issue that is stopping you.
What about the remote control unit, have you changed the battery recently?
Some useful info about using any operating car, if you have an with smoke or a caboose wit smoke your track gets dirty almost twice as fast as a non-smoking engine or caboose.
Another thing that gets track dirty quick is post war freight & passenger cars as they keep dirt on their wheels for some reason.
Lee Fritz