My ball which has a circle collider 2D component occasionally passes through my box collider2D object. Both objects have rigidbodies attached. I have set a high mass for my box collider and a small mass for my ball. Strangely enough, it doesnt pass through every time (maybe due to varying velocity). Basically i am making pong.
This is because the ball -at high velovity- will be before the box in one frame and behind the box in the next: so no collision has occured.
To fix this make sure the collision is continious or interpolated.
Set your collision mode to DynamicContinious for high velocity objects or Continious for fast objects. This does have a bit of an impact on performance if you have a lot of objects so just apply it to objects that move fast.
It turns out all i had to do was increase the thickness of the box collider.