Usual cause is insufficient water at the root - most plants will abort their flowers if water is in short supply, its a survival mechanism.

where are you Andy? And which Hibiscus? The indoor tropical kind, or the syriacus 'Blue Bird' sort that grow outdoors? I have had this problem with my Bluebird in the past. In my case it wasn't dryness, it was lack of summer heat. They need heat in the summer to flower before it gets too cold. I resorted to keeping mine in the greenhouse over the summer and outside over the winter. It worked until it became too large to keep in the pot!

The tropical Hibiscus will also refuse to open its buds if it is overwatered.