Need help with onMouseDown ()

Working in 2D, C#

So i was having issues with onMouseDown not firing in my project, I figured it was something else in the project preventing it. Then i started a new project with a fresh new sprite and script and still nothing. After looking at different ways online i can't find anything.

My sprite has a 2D collider, on the top of the Hierarchy. Tried Is Trigger (and not Trigger) and with and without a RigidBody.

I'm pretty sure this is all i need right?:

 void onMouseDown () {
 
     Debug.Log ("working");
 }

Any help would be greatly appreciated, Im just flat out baffled. It's probably something tiny I've missed but it's made me to a halt...

Thanks!