Depending on your recent winter weather, it's likely that the plants suffered some winter damage. True pampas grass (Cortaderia selloana), which is invasive enough to be considered a noxious weed in many areas of the country, isn't as cold-hardy as many people think. If you live in Zone 6 or colder, you may be outside the plant?s comfort zone.
Your grass may be a look-alike called Ravenna grass or plume grass (Saccharum ravennae), which is far less invasive than pampas grass but is reliably hardy only to Zone 6. Both sometimes have the problem of the older, main clump dying, with any new growth coming only from the younger edges. These and most other ornamental grasses require full sun. If they are getting too much shade, they may need to be moved to a sunnier location.