Hello Mark, a very common problem with plants that are not flowering is that they are too well looked after! In fertile soil with lots of nitrogen, plants tend to put on leaf growth rather than flower. Flowering can be a response to stress - the plant flowers to produce seed, by which it can reproduce and ensure survival of the species if it dies! Tomato fertiliser - which is high in potassium, can induce flowering. However I would just leave your acanthus where it is for now. Don't give it extra water. If the clump thickens up it will probably become more stressed and come into flower. I hope you're enjoying the leaves, which are very beautiful in themselves.
Emma
gardenersworld.com team