Me too re Agapanthus. Planted in big pot, only put 3 in. I’ve lost one and the other two only put out leaves. This is second summer. I’ve given them a severe talking to. I’m giving them until next summer to come into flower or they are in the bin!
I’ve fed them and followed RHS website advice too.
I sowed some annuals around them this year and they are doing well.
I had the same problem with a Peony, which was in flower when I bought it a number of years ago, then no flowers for four years. The talking too worked as it flowered this year!