Priority Bug
This has happened to me twice during the current cube season. I have spells in hand that I can play in response to my opponent taking an action and the game proceeds without giving me the opportunity to do anything. I've lost both games where this has occurred when I was in a favorable position.
In the attached screenshot, my opponent was allowed to resolve a Sheoldred while I had Despark and Counterspell in hand with 4 mana available. The following turn, my opponent was able to attack me without giving me priority.
Not sure what is causing this to happen, but it's very frustrating to lose games like that. Playing in full control is a workaround for this, but is very tedious to do.
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[Deleted User] commented
Read Myrel! She doesn't just make soldier tokens when attacking, she also doesn't let an opponent cast spells or activate abilities during her controller's turn.