My Agapanthus like to bloom better and better as they get crowded in their pot. They don't do well in the ground here, maybe as Lin says because it's too wet in the summer.
So, I'm saying first, a moisture-retentive potting mix might not be the greatest thing, maybe mix in some Perlite or other airy stuff while you have the chance. Second, don't be surprised if they don't bloom this year. Mine always seem to pout a bit when I have to re-pot them, so I just leave them alone and let them get really cozy in that pot before I do anything. All I do is each spring I add some new potting soil to the top of the pot, amended with alfalfa pellets and a tablespoon of pelleted fert, and wash it down into the pot with the hose. They seem to like it.
Donald, yes I agree, they don't seem to like full sun here, although in California they seem to do fine. Maybe it's their air pollution? Mine get morning sun only and other than through hot, dry spring weather I don't water them either.
There are two types in this pot, a tall one and a shorter one and they bloom at different times. These are the short ones.