Eccentric Spacer Problems

I've been trying to build a belt and pinion carriage as shown in the example build video ( http://openbuilds.com/builds/v-slot-belt-pinion-example-build.97/ ), but I'm having problems getting enough pre-load of the wheels against the rails. Even with the eccentric spacers turned all the way in, the carriage is still loose against the rail.

I've attached some pictures and video showing my build. Perhaps I'm missing something?

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Eccentric Spacer Problems