Slitherine

1st of all Caster of Magic is excellent and has remade one of my favorites. It's very challenging now. I haven't won yet.

I have a problem is that suddenly my healing spell no longer works in combat.

What's going on

Could you provide some more details?
Which unit are you targeting? Is it affected by effects that would prevent healing like Mystic Surge, being undead, or Raise Dead?
Is there a message that says the unit cannot be healed?