Maybe solid clay is not ideal for the agapanthuses, since they like free-draining soil. I saw this in an article from The Telegraph:
¨...Most critically, make sure they have good drainage. In pots he uses two parts of compost to one part of coarse sand or gravel. Where they grow naturally in South Africa, on cliff tops, crevices and ledges, there is precious little soil.
Restricting them makes the plants flower sooner, but when they mature over several years in the open ground, they get to their full size.
In containers, even when they are fully mature they will not get to their ultimate size because of the bonsai effect, unless it’s a very large container or a dwarf agapanthus. This is why plants directly planted in the border can take years to flower reliably, but when they do their flowers are more spectacular.
Thirdly, feeding is key. High potash feeds, such as the one Hoyland sells for agapanthus (which is 30 per cent potash and trace elements), applied every three or four weeks in the growing season, with a general feed twice or so in the summer, will keep the flowers coming.¨