why didn't OnMouseXXX get called?

I have a bunch of objects with colliders on them and they all receive OnMouseDown/Up etc callbacks. Except one.

• The collider works fine, for rays and physics.

• the script with OnMouseDown/etc is on the same object as the collider (not a child or parent)

• it's not a trigger or in any ignored layer

• visually it looks fine

• it used to work, now it doesn't. The code is the same.

The object is a prefab comprising: root with RigidBody child sprite with box collider and script, rotated to face upwards (camera looks down)

The manual says "OnMouseDown is called when the user has pressed the mouse button while over the Collider.". Not much to go on to figure out why it isn't getting called.