The top of your bars are expected to be crumbly, but the streusel should bake nicely into the filling. As for the bottom, there might be too much flour or too little butter. Both of these ingredients will affect how well your crust sticks together. Be sure to measure and use the proper ratios. Pressing the crust down evenly into the dish will help it stay firm after it bakes. I would recommend using the bottom of a measuring cup to press the crust for best results!