Despair + Impending Doom

I'm trying to test Impending Doom slotted in to Despair curse as I find it an interesting combo. I also slotted inspiration, spell echo and cursed ground. I seem to be able to kill groups of mobs fine but one it comes to the last mob, they don't take damage. If I move to the next group, same thing happens. I then tried on a mini boss and since it's a single mob again, they also don't take damage. What am I doing wrong?

Nothing wrong, last mob is on cursed ground hence despair is applied to him. ID won't trigger until cursed ground disappears.
You may cast spell one more time, so this last cursed ground disappears, and ID triggered.

I was definitely recasting Despair to make the first cast expire but the mob was not taking damage. I would think Spell Echo would still cause the mob to take damage since it's casting twice already anyway. So I'm not sure what's going on with the mechanic. Any thoughts?