I'm trying to model a race car suspension. Part of that involves suspension struts. The struts are a hex tube with heim (spherical) joints at each end. I've mated 5 of these struts, at each end (10 mate locations, 20 mates) so far, but the last one refuses to mate with the error "Please select from two different subassemblies...". The heim and the frame ARE different subassemblies, so I don't understand why I'm getting this error. My VAR wasn't able to offer a solution, so hopefully someone here can.

I added an image of the two parts I'm trying to mate - basically the ball of the heim to the slot in the frame. Basically it needs 2 mates - a face coincident and a slot distance, but I can't do either.

Thanks in advance!