0-6-0 with sloping tender problem

[NKen]

I just purchased an 0-6-0 with sloping tender for N scale.  Everytime it goes through turnouts or crossings it stalls.  All of my other engines run fine through the turn outs and crossings but for some reason it loses power when the tender is in the turnout while the engine wheels will be on straight tack which tells me something is wrong with the front wheels.  
Any suggestions?

I think I found the problem.  A screw that secures the left rear wheel contact will not tighten or loosen.  I submitted a service request with Bachman.