Shields add to your Armor Class. Your armor class is a combination of your characters armor, ability to block with a shield, ability to parry with their weapon, and the fact that people don't stand still in a fight.
So if you do not roll high enough to pass someone's AC, your attack fails. The game calls it a 'miss', though it can be due to missing, the enemy dodging, the enemy parrying, or the enemy blocking.
Mechanically, 5E does not differentiate between ANY of that. There's no 'This is your armor AC. Then this is your shield AC. Then this is your AC for moving around. And this one is for parrying'.
So there's no way for the game, or the devs, to decide what's a miss, a dodge, a parry, or a block.
They could put in animations each occurring a percentage of the time based on what percentage of your positive armor class modifier is from armor, dex, and shield respectively.
