I bake for a small cafe in my town. We (myself and another baker) are having problems with two kinds of muffins--blueberry and peach cobbler. Both seem to have the same problem when we bake them. Each have a streusel topping and when baked, both kind of sink in the middle (actually looks like a small hole on top). What are we doing wrong? This has been a constant problem, not just one time. Any ideas?