Well, the ones I have raised flowered every year. Do they seem to be healthy otherwise? Did they suffer any frost damage this past winter? They do form flower buds the previous fall, though frost consistently every second year sounds outre! Another thought: do you allow seed pods to form? Seed formation can take enough energy to reduce or eliminate bloom the following year. Pruning too late in the fall will also stop blooming, though that's also odd if every other year. I hope at least some of this helps!