Rose of Sharon (Hypericum calycinum) flowers on the ends of new shoots, so the more new shoots you can encourage the plant to make, the more flowers you'll get. But bear in mind that, although they don't mind shade, they flower better in sun or partial sun.

The best time to prune these is late winter or very early spring, and they should be pruned hard each year to get lots of those all-important new shoots. :)

My Rose of Sharon is a different plant altogether from yours (mine is hibiscus), but I agree with above. It's lack of sun, not so much pruning .

A photograph would be very useful here - Rose of Sharon means different things depending where you are, but in the UK, generally, Rose of Sharon, although originally assigned to Hypericum calycinum, is also commonly used for other varieties of Hypericum such as 'Hidcote', which gets easily 6 x 6 feet. If you have Hypericum Hidcote, that does need some sun, whereas H. calycinum will grow in shade - but doesn't get 3 x 3 feet.

Establishing precisely which plant we're talking about would be very useful.

Good point, Bamboo. I always think of it as Hypericum calycinum, and the 'Hidcote' variety as St John's Wort, but others may not, so a photo would be handy.

St John's wort to me is the group of little wild ones. I'd have expected Hidcote to be taller than that after six years though - I wonder if Bermiew has the dwarf variety that does get to abut that size? In any case north facing shade isn't the best spot for it. If you want flowers you might be better moving it this winter and replacing it with something that would be happy north facing.

There are an awful lot of varieties of Hypericum, and there's no way of knowing what Berniew actually means by 'Rose of Sharon' - might not even be a hypericum at all.

So it appears to be stalemate until we see a photo...

Or get a proper botanical name...