I found that I had to use:-
bolt - front bracket - brass washer - axle - brass washer - rear bracket - lots of washers - nut.
The bolt has a shank to it, and the axle and both brackets need to be on the shank to ensure that the axle is level. If you add a washer between bolt and front bracket, you may find that the rear bracket is resting on the threads of the bolt, and not the shank.