I have a Bachmann 2-8-4 Berkshire Steam Locomotive that worked file when I fist got it, but now seems to having a hard time pulling almost any freight. When I first got the locomotive it could pull 8 of the Walther's heavyweight passenger cars without a problem. Now I can pull two at full power (28). I've tried resetting the decoder to factory setting and the problem remains. I'm not sure what else it could be. There doesn't seem to be any hair, dirt, grit wrapped around the wheels or anything.  Suggestions on things to try?