To state the obvious, something happened between disassembly and reassembly. This link takes you to the Service Manual for your mixer:
++http://www.kitchenaidparts.eu/documents/pdf/service-manual/classic-svc-manual-new-email-version.pdf++
Things to consider:
The brushes have a chamfer on one corner. This must clear a stop in the brush holder. Take a flashlight and look into the holder and make sure the brush is aligned before inserting it.
Are all the wires attached to the proper tabs on the control board?
Is the link that pushes on the control plate properly positioned. This link also actuates the on-off switch on the control board.
The Service Manual will help you make sure that everything is in its proper place.