Try cleaning the tracks and the engine wheels, and this should solve the
spart problem.
A sponge with a scotch brite scrubbing pad works well on the track, or
a foam sanding block rubbed lightly on the tracks, in the direction of the rails, and then wiped with a soft cloth or paper towel.
Even the nickel track will get an build up of oil and dirt, and brass track
simply corodes.
have a great day.