Why didn't my ornamental fountain grass put up plumes in the fall?
Ornamental Fountain Grass Didn’t Put Up Plumes in the Fall
Certified GKH Gardening Expert
I am not certain as I have not heard of this happening before, but I would recommend having the soil tested. I believe that the plants may be low on phosphorous or too high in nitrogen. This can cause a lack of flowering on other plants, which is essentially what happened here.