I need a lot more information to even venture a guess. Some common reasons are: lack of sufficient sun; too much of the wrong fertilizer; virus.

They like fast draining sandy soil, moderate nitrogen, a little extra potash, light on the phosphate, at least a half day of direct sun, and moderate crowding if in containers.

Basically feed the non-flowering ones with Tomato fertiliser at half strength.

Thanks for all the advice, will implement some especially the feeding.