I noticed that when lowering my z axis, the nut was coming out of the x axis injected molded idler end. Unless I pushed down on the end before my print started, to reset the nut into place, it would cause the first layer not to stick well.
The smaller piece fits against the bottom of the nut with the nubs up, and the bigger piece holds it in place.
This is for a Prusa i3 clone with injected molded parts. The z screw it a 5mm threaded rod.