You might have had more success leaving it in a pot! Agapanthus like to have their feet packed in really tightly - as I found out after splitting one up that had been in a large pot.....it took a couple of years before it flowered in the ground.
Water it with something like tomato food which promotes flowering - a bit late for this year I expect but feed again in the spring when it appears & you might have better luck next year.
I was going to say the same as Robinia, I doubt that they will ever flower in the ground, mine were nearly bursting the pot they were in before they started to flower, and my Fathers are just the same, for some reason they love to have their roots really cramped.