Assuming the mechanism turns freely, the easiest thing to check is the orientation of the drum/brusholders.
If you take out those little drums, and work the reversing lever, you will see a small metal finger come down into the drum chamber. The drum with teeth sticking out radially goes on the side with the finger. The second drum, with recessed teeth goes on the other side. The teeth engage eachother. When they are engaged, where the teeth mesh, a copper segment on one drum should match up to a plastic segment on the other.
Look at the contact fingers. each finger assembly should be in contact with a copper segement on a different drum, and should only be contacting one drum. The contact should rest on a plastic segment on the alternate one.
Sometimes the drums get stuck between operating positions. Working the reversing control lever a couple of times usually clears this problem up. If this problem keeps occuring, then there may be a burr on one of the drums, or the mechanism needs attention.