Why does my trigger-collider receive Enter but not Stay or Exit?

Why would a mesh collider call OnTriggerEnter but not OnTriggerStay or OnTriggerExit? My coworkers and I are all baffled; all of our guesses have been disproved.

The design for my project involves a turret with an area of effect described by a symmetrical triangle (mesh from our modelers), with the point at the turret and scaled along the turret’s trajectory to also touch the cursor.

Follow that cursor!

When the user moves the cursor (mouse or touch), the area of effect rotates to look at it and scales to span the gap between the turret and the cursor. While all of this is happening, I need to know when collisions with other stationary things start, continue, and stop.

For completeness, I should mention that the same modeler made both the triangle and another mesh that I position by the cursor. The other mesh reports collisions just fine, but the triangle only reports OnTriggerEnter.

Looks right to me...

If I put a BoxCollider on the triangle, it works fine.

The other model is positioned near the cursor by placing it at the same position as an empty GameObject childed to the triangle. Since it receives collisions fine and collisions are supposed to require motion, I tried making another triangle and positioning the original triangle according to an empty GameObject on my new triangle (whew!). It didn’t help. (I would include another picture, but I’m at my limit.)

So… What can I do to convince my triangle to work?

you should read the documentation. Unity - Manual: Mesh Collider component reference

first off for 2 mesh colliders to colilde they both must be marked convex.

You also need to look at the very bottom of the page and line up what kind of collider the semi-circle is and what kind the triangle is (for example one of them MUST HAVE a rigidbody). In order for this to work.

Anyways your answer is all there in the documentation on mesh colliders.