You did not mention how old the junipers are and the site location, sun or shade. Junipers usually look good once planted and after a while decline due to lack of sunlight, winter injury, vole damage http://extension.umd.edu/hgic/voles-groundcovers, possible diseases, poor drainage, etc. Junipers grow best in full sunlight and a well drained soil. They will thin out and brown due to lack of sunlight. They also may have been affected by our cold winter and we have had several reports of browning junipers.
All you can do is remove any fallen leaves around the plants to improve the air circulation and prune dead wood. The plant will put out new growth where there are green leaves only. You can mulch with no more than several inches of mulch to keep weeds down. Be sure to keep away from the stems. Monitor for new growth this growing season.