Why not give it a feed, say with a tomato feed or any other water on feed that encourages flower & fruit production. That might do the trick.

Hi, the Hibiscus normally grown outside in the UK, are varieties of H sinosyriacus, or H syriacus, unfortunately they, {as all species do} need a long hot summer to flower well, which unfortunately we haven't had for several years, they should flower in late summer to autumn, I had 1 for several years in the garden, that also never flowered well, so I dug it up, and put it in my greenhouse, the following year it was a mass of flowers.
Outside they should be grown in humus rich, moist but well drained, neutral to slightly alkaline soil, in full sun, Derek.