Certified GKH Gardening Expert
No, these will bear both sexes in the same flower. They can fail to flower for a few reasons though. Trimming them at the wrong time, not having enough nutrients in the soil, and excessively cold temperatures will delay, or even stop flowering.
This article will help with the care of this grass: https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm