Why is my agapanthus not flowering? What I've learned after years of growing these blooms as a professional gardener

Blue flowers of agapanthus in a garden border next to a lush green lawn.

Agapanthus, sometimes referred to as African lily or Lily of the Nile, are stars of the summer garden. Over my time as a gardener, I have developed a growing appreciation for them, particularly as they tolerate heat so well (as a plant native to the Cape of South Africa).

However, while they are relatively easy to grow, they can often have an off-season in terms of flowers, sometimes producing only lush green blades without a flower stem in sight.