humm. Mine are blooming like gangbusters. They are in a planter under my maple tree where they get dappled sun and bright shade. All I do is deadhead and pinch off a leaf now and then if it's hiding the flowers. I gave it some slow released fertilizer when planting. They need bright filtered light. Soil should be kept consistently moist, not mucky or too dry.

Too much high nitrogen fertilizer will prevent flowers & so will deep shade. You haven't provided much info as to growing conditions but these are the two biggest reasons they will not bloom.

Maybe you didn't plant them as early as you might have done? Sometimes they can seem to take for ever to get started. Can't think of any other reason.

Not sure where you are inUK, but my begonias are not bloom in either in the north east. Rotten cool summer to blame I think.