i have a later flowering ice blue agapanthus, and this year it only has 2 flowering stalks, last year it had none, and the year before it was full of them. the flower heads are huge, as big as a football. i leave them on for as long as they stay upright, like robinia i like the interest they give.
are they temperamental plants? with mine doing so differently over the last three years (i've had it about 6 years) i wondered if the weather conditions which have been so changeable have had an impact, even though it's in the same pot and in the same position.