It's hard to say. You're certainly well within their hardiness range, so even a tough winter shouldn't affect them.

Things which can kill or stunt them include not enough regular water, not enough sun and failure to cut them back and rake out the shaved crowns. They like moist to wet soil, full sun and must be cut back in late winter. After cutting them back, they like to have all the leaves and other goop which collects in the crown raked away.

Ah, I had a number of grasses with the same issue. They were great last year, very healthy. When spring came around (early this year), many of them were not growing, while others nearby were growing vigorously. I finally got around to cleaning up in early June, and by that time most of them had a few shoots. When I launched into cutting off the old stalks, I discovered immediately what the problem was, voles had chewed/tunneled into the plants right at the crown level, and had shredded a lot of the plant tissue that would have contained the new eyes, and actually made nests in there. From the outside, the plants looked perfectly normal, but at ground level they were destroyed. About a dozen plants affected, all of them have now survived, but some have only a dozen shoots, so basically are starting over from scratch.

"They like moist to wet soil"

Interesting to hear you say that, as I barely water mine. I by 'barely', I mean once every 4-6 weeks. And they're in blazing full sun, doing great.

Differences in soil water retention, perhaps?