Its Uncommon but it happens. You will Have the mana to play a spell, And for whatever reason not even have the ability to play it.
Sometimes a stack happens and with no modifiers on the board for casting instants, you will simply not have a chance to respond. When this happens it makes the game feel unfair, and rigged.
This Must be a High-priority fix.
Its literally counter intuitive to the game/mechanics/rulings. And it’s literally cheating when it happens.

For Example:
imagine going to tournament irl & you play a Planeswalker and the opponent doesn’t get to play negate. The judges declare that it just instantly enters the battlefield and that it can’t be countered (the card does not state “Can’t be countered.”)
This is literally what is happening in a sense.
And the game is like the judges.
Personally the last this time occurred: i was playing against nothing but an Ovika triggering on thirst for knowledge which was triggering Arcade Bombardment which was copying abrade. Despite having 10 mana all of color. I was presented no opportunity to cast Offer Immortality in the stack (instantly and favorably resolving for my opponent) resulting in the immediate loss of a game I otherwise 100% would have won.