Check the code Update() method in the class
It iterates the collision actors (updating their motion physics for gravity in the process) and checks if there are any collision objects in the collision grid that occupy the same space. If so, it invokes the OnCollision() callback for the collision actor.
I didn’t write that code myself (hi, Dylan!), and there are definitely ways to make the collision detection process more performant and easier to use; we are working towards that goal.
In your case, my guess is either the collision grid is null, the actors we not added to the collision world, or the bounding volume (rectangle for this API) is not the correct for the collision actors in your setup.