What you were expecting to happen, and what actually happened?
The screenshot will show that Gorgotha had 15 mana, and needed 15 mana but his spell was not ready.
On his next action he took a 3 skull match, dealing damage to my bone dragon (and renewing his freeze) but then the spell instantly became ready, although no other matches formed from the board refill.
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So I moved that skull across to the left (matching 3 brown) and a skull cascaded right above that yellow.
I should have guessed, of course. Anyway, once he matched those three skulls, his spell was ready
This image was at the same time, showing that he needed exactly 15 mana for his spell, and that the spell image was not the one shown.