Happens to me too. It really sucks when you're casting the spell to interrupt a heal or something that is going to deny your kill... and it doesn't work.

Sometimes when I use Heart of Life (I think?) while moving, the Demigod will stop and use it. Other times, it will keep moving and won't use it. And sometimes, it keeps moving AND uses it, without me having to press the button more than once or issue a new move order.

Hopefully this gets fixed soon. I play Sedna a lot because I really enjoy providing powerfull heals for my team, but it pisses me off when I lose a team mate just because of this bug. In fact the worst incident just happened to me about an hour ago. I was trying to keep a friendly up (Oak), as Sedna moved into range she just stopped and watched as he fought for survival. I quickly took note of the problem and reset Sedna's state by moving her around then re-casting the heal, still nothing. My ability to heal didn't come back until after Oak was dead.

Now I only assume that this is a bug. I do know that Sedna has an aura that doesn't allow casters to heal while in the vicinity. Coincidently, I've NEVER played any of the Forces of Darkness Assassins/Generals, so I don't know if they have a simliar ability.