Non flowering hardy hibiscus

The one thing Hibiscus cannot tolerate is to dry out. Make sure you mulch the plants in spring with a good composted manure to help the ground stay moist and to feed them up at the same time.
I have just had two Hibiscus shrubs flower for the first time in one of the gardens I work in. The owner didn't even know what colour they were they had not flowered since she moved in several years ago. I mulched and made sure they were watered during the really dry spell we had earlier in the year.
Shade shouldn't matter too much as long as it is not too heavey. My Hibiscus is in shade and flowers fine.

Mulching hibiscus is a good idea as Jess says. But what's the name of your hibiscus? Some flower a month or more later than others. Sounds like you've got a late one and with our non-summer it's unable to enjoy enough strong sun to trigger flowering.

I get the impression they enjoy sun and warmth. At the moment the gardens in Hungary are full of hibiscus in full flower. Some of the bushes are huge. We get a lot more sun than UK.