Why isn't my canna flowering?

As far as I know Spritz it is unlikely to flower now as they need 6 hours of sun a day! Down here they are all starting to die back.
I might have found a reason for the non flowering of yours though!!
It might be an agricultural canna rather than a horticultural one!. They are grown for their rhizome which is full of starch and tend to be sterile.
Have a look at the Wikipedia entry on Cannas

Hi Maple - oh dear. I did hope it might still flower... it was bought at a reputable Garden Centre, so surely not an agricultural one from there? I suppose it might flower next year if I can overwinter it.

Did your canna come in a sealed pack with a picture on it? We had a couple from a local canna nursery giving a talk at our local Garden Club last night. He was critical of some outlets who sold cannas in this way because what was pictured on the pack bore no resemblence to what the rhizome in it was capable of producing. Probably more of a problem with the supplier than the Garden Centre itself.

No, it was in a smallish pot with a plant label - no name though. Next time I go over to the G.C. concerned, I intend to complain! I am very disappointed. It was not cheap!

I've heard that during it's first year it does not flower.
So maybe the problem is in your expectation, not canna's ability to flower.