All I can think of is the possibility of some sort of disease. This plant is probably the easiest grass to grow, in almost any kind of garden environment. Real drought is the only thing I've seen slow them down. I have them in several different locations, from full sun to full shade, and they grow like weeds.

nothing to loose .... I would dig them up, take a sharp knife & slice them in half (or more), then dunk them in a bucket of water to get rid of some of the mixture used for potting. I have seen some plants (grasses included) fail to grow their way out of their prison of pot bound roots & disgusting pro-mix type potting mix.
Once you have rinsed off some of the potting mix then loosen up the root ball - you can be fairly brutal - & replant.
Then ...... fingers crossed ;o)
a.