Spell sneak attack doesn't seem to be as forgiving as regulra sneak attacks. I haven't played assassin, but from what i hear of that class, the spell sneak attack works more like the death attack.
In my experience, your best bet for landing spell sneak attacks, is to first disable/stun the target, with bewildering cloud or something. Or be undetected (and hasted). Then if you're close enough, your spell sneak attacks should happen 100%. Otherwise its very hit and miss.