Ok i have the shell off,  When i apply throttle to the engine the engine revs up but doesnt move,  i poke at the motor when its in the forward position and it moves jerkingly,   while in the reverse position it runs like it should, very strange....and i tried dc.   only has enough power to start up the loco

I just purchased a new ALCO S4,  WM #146 to be specific with DCC and Sound.  I put it on the track and from the box it ran perfectly, at some turnouts on the layout it shot-circuted, but other than that it was fine,  but then the engine starts to act up.  It starts to refuse to move until i push it.   Eventually i got it to go,  but then it starts to slow down when going in forward,  i set to full speed but the engine is at a crawl and now at its current state cant move forward, but needs to be nudged a bit to go in reverse,  what is the problem? I like this engine alot.