Looking at your photo, there is no sign of any attachment points for extra pickups to the wheels. So here is how I think it should work, I'm pretty sure I have a similar loco.
I think that the entire bogie should be live, but that the wheels on one side should be insulated from it. So pickup is via the axles, which are only connected electrically to the wheels on one side. The wheels on the other side will have a plastic insulator of some kind.
To test this, get a multimeter and test continuity from each axle to its wheels. If I am correct, you'll see an electrical connection to wheels on one side only.
If this is true, then next test continuity between the body of the bogie and each axle. If this fails, then you probably have a build up of muck acting as an insulator around the axles. Cleaning should fix this.
If there is continuity between the axles and the bogie, then the fault will be the connectivity of the brass pin to the bogie. Test this with a multimeter and clean if necessary.
Hope this makes sense....
Phil.