I'm trying to make a benchpress for my gym. Everything is done except the uper seat base that can move with respect to channel attached bellow the main base. When I mate square tube (seat support) faces coincident to the inner faces of channel than everything works fine. Three components should have to move according to condition channel, seat base and the support but when I try to move seat base it says "component is fully defined." I spent many hours on it but now I quit. 

I'm attaching my subassembly Please help me out guys try to solve its motion.

Thank you so much for any kind of help.

Parts names:
Channel that will move = black one
Seat support = the one which has many holes
Main base = fixed one
seat base = inclined one