I noticed that, on Sedna, sometimes I press the hotkey for Heal (3) and target it on a highlighted ally and it doesnt go off. Once, I tried placing it again 2 more times and it still doesnt work. Finally, it works the fourth time.
It only seems to happen when the ally is within range (Otherwise Sedna gets within range and heals) and happens fairly often, which is very annoying when heals are critical.