I have a lot of trouble mating a sub-assembly to an assembly. (However, I have seen this with a single part within an assembly). The sub-assembly has (minus sign) next to it and shows no mates inside assembly. I proceed to try and constrain in the assembly by mating with simple mates. I usually try and mate planes together, but sometimes faces to planes if they are not perfectly straight to each other. Regardless, nothing seems to work. The first mate usually works but I am met with "...can not be mated...would over-constrain...etc..." for the others. Seems straightforward if I was dealing with anything else but SW.