Before you consider buying a dethatcher consider that rotting grass forms top soil. But to do that you need the proper conditions for the clippings to rot. If you have an acid soil, clippings will accumulate much faster than they can rot.Tthe reason is that an acid soil acts to preserve the clippings. It's a natural pickling process that we also take advantage of to preserve foods.
Sweeten up the soil if it's acid and the clippings will rot faster and you may not need to dethatch.