OnTriggerEnter2D() is never being called

I am doing an experiment trying to use a Collider2D (specifically a Collider Circle 2D) component on my game objects in a 2D game for simple collision detection. Wanting to see if this is faster than doing my own logic.

Right out of the gate, though, I am surprised that thus far I have not been able to get any collision contacts to fire at all and wondering what obvious thing here I might be missing.

• Both GameObjects have a Collider Circle 2D that is enabled and active.

• Both colliders are set to Trigger mode

• Have a script component on one of the GameObjects that has implemented both OnTriggerStay2D and OnTriggerEnter2D

• Have confirmed that both gameobjects have the same Z value (not even sure if a 2d game allows z value to change)

• Have paused the game in the editor and its clear that the colliders are definitely overlapping

• Also tried adding a RigidBody2D component to both gameobjects after reading some other posts but still never detecting trigger collision.

No OnTriggerStay2d or OnTriggerEnter2D is ever being called even once. What am I missing?