Certified GKH Gardening Expert
There are a few reasons why this could be the case. It may not be old enough: They will not flower during the first, and sometimes second year.
They are too compact: This is one of the reasons that I believe that the plant is not flowering. This plant has a long taproot and will not be suited for container life. Unfortunately, they do not transplant well, either.
Too much nutrients: This plant thrives in poor, sandy soils. Nutrient rich soils will cause stretchiness and no flowering.
Too much moisture: These plants cannot tolerate wet or compact soils, at all.
This article will give you more information on the requirements of Eryngium: