I assembled some parts to be two slider-crank mechanisms with almost the same constraits, but there was a problem puzzling me. In the slider-crank mechanism.prt, there is a subassembly, and the mechanism can't be moved. Whereas, in the slider-crank mechanism_modified.prt, all the components are at the same level, and the mechanism can be moved. Why? How can I make the former mechanism effective as the second one with the subassembly?