Certified GKH Gardening Expert
If it is a large and old patch, it may be that they are in need of rejuvenation. The middle of the patch starts to die out if the plant is not regularly rejuvenated.
All you need to do is dig up the patch and divide the plant and replant part in the spot where you dug it up from. This forces the plant to rejuvenate and it will start to perform better. This article will help with dividing the plant: