Often this happens if the crumb topping is over-mixed or if the butter was much too hot. Over-mixing will leave you with a pasty, batter-like mixture rather than the crumbly topping you are aiming for. Using very hot butter can cause the sugars to melt, leaving you with a greasy and not crumbly mixture.
Get more content like this and the inside gist on all the fun things that happen at Healthsoothe straight to your inbox when you Subscribe to the Healthsoothe newsletter. Do it now!