I have the following spell/macro binded in "Clicked" on mouse button4. Also tried button3 and button5.

/stopspelltarget /cast [@mouseover,help,nodead]Power Word: Shield;[@target,help,nodead]Power Word: Shield

If I bind it on the keyboard it works.
If it's binded on any of the mouse buttons I can't cast it on party members/self unit frames, nothing happens. On default Blizzard unit frames it works as intended.