I think a cool late winter/early spring retarded growth. I certainly saw the same with grains and clover. The latest coming on that I remember. Actually thought for a while maybe my soils were messed up but as soil temps warmed, plots improved. Same occurred with new yard I planted , very slow growth of grass, thought I was loosing my touch, and I have a reputation to uphold in the community of being able to grow on a side of a rock!