Zombies not breaking down doors

Zombies will walk up to my doors and just stand there like they did when they would break down doors... But the doors don't break. I am on hard difficulty but they still do not break them. I play on survival with no mods / texture packs. ( When the bug appeared I got rid of my texture pack ) I also play with no armor or iron doors to make it harder, so this is a huge blow to my game.