Why is my Craftsman Model 315.108220 not allowing me to tighten the bl

I just removed the nut holding the blade and replaced the blade. After replacing the blade I was careful to put the nut and two washers back in the same orientation under the nut. The nut will not fully tighten. The saw blade is loose and has some wobble. Now I am unable to take the nut off as it turns without unscrewing while holding the lade fixed. What is wrong?