I have a grass garden of different grasses like Goliath, Cascades, Karl Foerester morning lights etc. and most of them are growing in around the outer edges and nothing in the middle. They kind of did this last year too, so my husband tried to loosen up and rake out the center so new could grow in the center but it didn't seem to help. Why is this happening and how can we prevent it. Our grass garden is only about 4 years old.