[Faraque]

This problem is a known problem. A mouseclick over a unitframe is intercepted by the WoW client. It's not treated like you would expect. You *must* use a keyboard keypress to activate an ability on mouseover on a unitframe.

To test what I mean, move your mouseover macro to button 1 of your main hotbar.
Move your mouse over someone in the 3D world.
Press the 1 button on the keyboard.
It will fire.

Now, move it over a target unitframe and press button 1.
It will fire.

Put it back as a mouseclick keybind.
Hover over someone in the 3D world and do the mouseclick.
It will fire.

Now put it over a target unitframe and do the mouseclick.
It will fail.

The only way to trigger a spellcast on click over a unitframe is to use an addon such as Healbot, VuDu, or Clique. (My preference is with Clique)

I hope this helps your understanding.