I'm working on an assembly that has 2 linear rail sub assemblies in it. The linear rail assemblies consist of only the rail and the truck which is mated so that it can slide up and down the rail. I brought 2 instances of these assemblies into the top level assembly and mated them to the frame where they are supposed to be. If I make one linear rail sub assembly flexible by editing it's properties then I can move the truck as expected. but if I make the other linear rail flexible as well then neither will move and a number of previously happy mates (for parts attached to the trucks) fail. These individual assemblies and their parts should be discreet. They certainly do not share any common mates or mating features. Is there a simple fix to this? can someone tell me what's going on?