My crumble topping is a foolproof crumble topping. It will turn out as the perfect crumble. A couple of other things could be happening as well. Do not cover the dish while baking. That will retain moisture and not allow the topping to crisp up. Also, you don't want a huge thick crumble topping. When it is too thick, the crumble just doesn't have a chance to crisp up.