If your crumb topping isn't crumbly you need to add a little more butter. Chances are you mis-measured the flour, but it is an easy fix. Start with 1 tablespoon of melted butter and mix that in, adding more if necessary. Don't add too much butter, though, as it will melt as it's baked. Alternately if your crumb is too wet you can add more flour 1 tablespoon at a time until the right consistency is achieved.