I have exactly the same problems as you, it would be interesting to read what answers you get.

Is it a pruning issue. If you're pruning at the wrong time they won't flower

Me too Michaella! We have one which was moved several years ago and now has the odd leaf (to prove it's still alive) that are about the size of your thumb nail. No flowers and not even a green bush, just a dead looking twiggy thing with tiny green spots of leaves!

Thanks for reply anchorman. But when is the right time to prune? Have never pruned as yet. As I said they look lovely and healthy but still no flowers. Seem to produce leaves on last years growth so reluctant to hard prune. Anyone out there got the answer?

I found this helpful( I kept a copy but can't remember where I got the information from)

They can be kept to suitable proportions by pruning them each spring.

They flower on this season's growth, so hard pruning that leads to the production of many side shoots makes them flower prolifically. Every sub-branch that springs from the main framework and is cut back will produce four new shoots in three or four months, each bearing clusters of flower buds by late summer. If these are cut away the following season, another relay of shoots and flowers will spring from the cut branches.

To make sure that hardy hibiscus produce plenty of flower buds, it is essential to plant in an open sunny position. Unlike many other sun-lovers, they also like plenty of humus worked into the planting sites. An annual dose of rose fertiliser promotes tough growth.

I had a supposedly hardy Hibiscus for several years, it usually got a few flowers on each year, but I had to move it, so I potted it up, and put it into a cold greenhouse, the following year it was absolutely covered with blossom, there must have been at least 200 individual flowers, so I came to the conclusion that, although they're hardy, the cold does nothing for their flowering capabilities, which , for me would indicate that they should be protected in winter, Derek.

You may be right Derek. I acquired these plants because I wanted a reminder of the times I spend each year in The Greek islands where my daughter lives. Hibiscus grow like weeds there and their winters are so much milder than ours. However since I don't have a greenhouse my plants are just going to have to survive the best they can. Thanks so much for your reply.