Q.Why isn’t my Mexican Feather Grass not growing ?
They were small when planted last spring in full sun and have not grown much. This is in our southern Arizona soil and are irrigated.

If the grasses are in a sprinkler system area, and getting more than the once a week watering, that could be a problem. It also doesn't like heavy mulch. We would suggest that you trim down the grasses to about 6", if they are now dormant for winter. This will take some stress off the plant, so it is not trying to get water up to higher portions of the plant. Then, only water them once a week, and not that if there has been regular rain. Let the water dribble into the plant area from a hose, and don't water them from an overhead sprinkler system if you can avoid it.
https://www.gardeningknowhow.com/ornamental/foliage/stipa-grass/stipa-mexican-feather-grass-care.htm