Canna not flowering - advice?

[Pippa]

I have a Canna indica 'Wyoming' that never flowers. I was given it as a freebie from a plant nursery as the tuber was so shrivelled they didn't think it would do anything. The first year, it just put up two tiny leaves and was less than 6 inches tall. This is it's fourth summer - it's been potted up twice and is now a lovely, healthy plant with lots of leaves about 2.5 feet tall, and with all the sun and warmth we've had this summer, I was really hopeful, but still no flowers.

The advice I've read said poor feeding, lack of watering and virus are the main three reasons for non flowering - but it gets plenty of food and water and has no signs of virus. Do I need to be patient and wait a couple more years or am I doing something wrong? It lives on my patio which is sheltered and full sun in summer and in winter it lives in the shed by the window.