This happened to me a ton in my one day of .76, it seems much better in .77, to be honest not sure if I have seen it at all, but obviously it's still occurring on some people systems (unfortunately).

I figured this one out today.  Your whole army can only have 1 spell being cast at a time.

If the unit casting the spell is hit, it is canceled.

If another unit casts a spell with a charge time both spells are canceled.

This happened to me as well multiple times. I would start casting summon earth elemental first with my sovereign, then a champion would start casting fire elemental on the same turn. The earth elemental would always be succesfully cast by my sovereign, but quite often (especially if he gets hit), my champion would not succesfully summon the fire elemental. (the cast button was activated again though, so if he wanted he could try to cast again, but there was no feedback as to why he didn't complete casting the first time)

I have noticed this also but could it be that initiative times get so crazy (which I love) that, perhaps, the spell is 20 turns behind and we never actually make it to it? Or should the spell be ready when the caster next gets his turn? Does the spell have its own initiative is the question.