When fighting the last bosses like Goro, Kintaro, and Shao Khan, hitting them often results in them executing some "flash block" where their body flashes white on contact. They don't even go into actual block mode. During this flash block, it seems as though they take normal physical damage, but they aren't knocked back or otherwise affected by the attack (ie, except for the damage, it's as if you never even hit them).
This is annoying, especially when you're trying to get out a good combo. You can be mid combo and they'll start flashing white and pummel you without actually breaking your combo. They just act as if you're not doing anything to them at all. Either there's a way to break this boss block or it's just a cheap way to make the game harder without having to invest more research into legitimate AI.