How old is the grass? Is it a really wet area?
I don't have any experience with pampas, but many other grasses need to be dug up and divided every 3-4 years or the center will die. Division will keep it from choking itself off and reinvigorate growth. It also appears from the picture that the dead growth from previous years may be inhibiting growth.
You could just dig up what is still alive, clear out the area of debris and replant. It will possibly come back if there aren't extenuating circumstances.
