I recently purchased online a Baldwin 4-6-0 Southern from Bachmann. Had troubles at first with the front truck, but got that fixed.  But now only the two front drives of the middle 6 wheels work - it gets stuck sometimes on my table top layout.  Looking at the bottom of the locomotive it looks like both the front 2 and the middle 2 of the 6 center wheels should provide power via gears.  Afraid to start tearing things apart.  Any suggestions would be appreciated.  Yes, I am a novice at this.

Thanks,
Dave