Devon,
United Kingdom 
I planted an acanthus 5 years ago. It flowered in its 3rd year but didn't manage to do so last year. I can't remember the name but it's a white one. It's in a bed of quite loose soil (clay with organic matter and grit added) and it gets afternoon sun for about 4 hours. Any advice is appreciated.
24 Jan, 2020
Answers
I have Acanthus mollis and I have been trying to dig it out for the last 4 yrs, as it has become a thug.
is your soil acidic? That could be the reason, though it has been a bit wet for the last couple of years. I am on freer draining soil with it being chalk.
25 Jan, 2020
Hi Bathgate. Yes and yet they are widely sold and I do see some fine specimens in other peoples' gardens.
SBG, I don't grow any acid-loving plants but I've never tested it and there is a lot of rain in Devon, so perhaps it's that.
I'm wondering whether I should feed it at some point.
25 Jan, 2020
a bit of food when it starts actively growing will be a bonus as clay tends to hang on to nutrients.
25 Jan, 2020
Thankyou and to Bathgate too.
25 Jan, 2020
I'm on acid soil and never have any problems growing acanthus.
A. spinosus is in full sun and flowers every year
A. mollis 'Alba' is in part shade and took a bit of time to get going, but now flowers regularly as well
25 Jan, 2020
I have had an Acanthus in my garden for years. It suits itself if it flowers on not. I live in Scotland so maybe this isn’t surprising. I gave a piece to a friend who put it in a very sunny site in his garden and it flowered every year.
25 Jan, 2020
It may have to do with temperature as these plants prefer a Mediterranean climate.
25 Jan, 2020