Is it getting at least several hours of sun a day? Is it getting any feeding?

I've grown Acanthus in shade under trees and they flowered each year. The mauve-y colours were less bright, but they survived. Maybe it's not mature enough?

They flower in the shade here, too, but not in very dark shade. I got the UK sun requirement from a previous string on the plant...now if only I could figure out the question number....