There can be many reasons that lead to a tough no knead bread but the most common one is not letting the dough proof (rest) long enough. The proofing time lets the glutens relax and adds gas (air). When this time is shortened the bread will be heavier and tougher. Another common issue is not kneading the bread long enough, but since this is a no knead bread you don't have to worry about kneading. In fact you want to make sure you are not overworking your dough altogether.