Instants & Flash don't cast during combat

[dungeonmasterforhire]

FYI. Instant and flash cards won't cast during declare blockers step of combat, like Ambush Viper, Evolution Charm or Winter's Chill.

[friarsol]

We need more information. Take a screenshot or something.

[ProphetSword]

Do you have the game set to actually stop during those phases to allow you to respond? If they are set to red, then you will not be prompted to do anything during those phases and it will pass right over it after blockers are selected.

Also, be aware that blockers are declared *before* any responses can be made. If you're attempting to put a creature into combat that you can block with (like Ambush Viper), you need to do that during the response step of the Attack phase.