Rather than selecting two flat faces to mate, select the flat face of the base and select the vertex of one of the linear edges of the structural member. Mate these two entities coincident.
Neeraj, the face on the extrude that you are using to mate must not be prefectly parrallel to the wheel caster. Try mating a different surface on the extrude and it should work for you. I got it to work for me.
without being able to check the model/assembly files - the first 2 things to come to mind are;
1) are either componet "fixed"?
2) perhaps faces you expect to be perpinducualr are not.