Each creature controlled by an opponent blocks only if it's able to do so as the declare blockers step begins. If, at that time, the creature is tapped, is affected by a spell or ability that says it can't block, or no creatures are attacking that player, a planeswalker controlled by that player, or a battle that player protects, then it doesn't block. If there's a cost associated with having the creature block, the player isn't forced to pay that cost, so it doesn't block in that case either.