object falls though another object

I have an object A, which has a box collider attached ('is trigger' not selected) and has a rigid body also attached '(use gravity' selected,' is kinematic' not selected) I pick it up and drop Object A on Object B (capsule collider attached - 'is trigger 'selected) to trigger a collision which is fine. Before I pick up object A and drop it on object B I want to rest it on object C, a rock. The problem is when I rest object A on Object C it falls through it. On object A I have tried turning on 'is trigger' but it falls through the terrain. I have tried it as 'is kinematic' this works but when I pick it up I can't throw object A properly anymore, I have tried attaching physical materials, messing with mass and drag size etc...but no joy. I’m sure its a very simple solution… how do I make object A sit on object C and not fall through it?? thanks