Floating a part in a Sub Assembly

I have parts in a sub-assembly. That sub-assembly is locatedin another sub-assembly. The parts are currently fixed, and I wantto float them and then add some mates.

I do not get the option to float the parts after a RMB selection.Both of the sub-assemblies above these parts are not fixed. I candrag the subassembly around and move it as a group.

Am I being stupid? Any ideas?

Thank you.SolidworksAssemblies