I also had a problem with this wheel.
Do you see the notch on the left of the base?
Turn the wheel so the marker on the wheel with the square is in line with the notch, then reassemble the moving part with the head and rollers.
This might help.
see also my struggle 
https://forum.dccmuseum.com/t/dcc600-new-gear-and-belts-and-some-problems-on-the-way-to-succes/934/2
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