It could one of several things, all fortunately minor. First off, check the locomotive wheels. They probably need cleaning. My favorite method is to take an old piece of t-shirt, soak it in rubbing alcohol and place over the tracks. Hold the engine with one truck on the cloth and the other truck on the rails, increase the speed with your power pack and the wheels will spin themselves clean. It may take more than one pass the first several times until you get shiny clean wheels.
Next, check all your track joints. Make sure the joiners are tight to each other.
Finally, check the wiring between the power pack and the track and make sure there are no loose connections. You don't state if your layout is wired for single or multiple train operation. If wired for multiple operation it may not hurt to solder each block connection.
These simple steps should improve your layout without giving you any more headaches.
:D Joe P