Bit of a myth really. Agapanthus do not really like being crammed in together, they flower better when grown in well fed, sunny places. I would either plant these out, or put them in a bigger pot in fresh rich compost and feed them.

Thanks Owlboggy will do as you suggested

Monty, on one of his programmes is still telling people that they need to be confined, no wonder everyone is confused! If you think in the wild they would have space, I don't know why they would need confining. Unless they are like Pelargoniums (old-fashioned pot geraniums) who everyone was told to keep dry and pot bound and they would flower better. Of course they would! Trying like mad to set seed before they died!