I'm guessing this is a Hybiscus syriacus variety? It's often one of the last shrubs into leaf, though I must say yours is extremely late!.

Though actually a quite winter hardy shrub when dormant, unless it's in the warmest, most sheltered spot for as early as possible shoot growth, many people struggle to barely get it to flower satisfactorily in this country before the season is closing in and autumn leaves are starting to colour.

Cooler areas are best growing it in a container and getting early growth underway from March and then moving outdoors as early summer progresses. Bringing the season forward like this at least gives you a chance of flowers.

Sheltered sunny London gardens and the south west (Cornwall) could be fine too.

Some named forms of this species perhaps perform better than others.

What is the name of your Hibiscus?