Have you got it in a sunny, free-draining situation? You can prune in summer, after if would have flowered. You could feed in spring with a tomato fertiliser, or sulphate of potash. Although it's not advisable to plant in the same place, it wouldn't just affect the flowering. If you try all of these things to no avail, then I would buy another plant and try again if you still have the will to do so.

have you been feeding it, and if so, how often and what with? They like poor, free draining soil in full sun.

Thank you jimmythe one and bamboo. The lavender plant is in full sun and yes I have fed it occasionally with a weak tomato feed. Might take your advice and replace it.

I'm willing to bet its the feeding - Lavender really doesn't appreciate being fed, flowers much better without fertiliser and in poorer soil.