Do you prune it? The flowers tend to form on last years side shoots. Try feeding with tomato food, but you won't get any flowers til next year now.

Hi, welcome to GoY, I agree with volunteer, but also if you live in a cold part of the country,it could be getting knocked back each year by frost, as they are only frost hardy, and may need some protection in the winter, they would normally flower prolifically, which is what they are renowned for, Derek.