Blueberry Not Fruiting

Hi All.  You were so helpful last week when I posted, I would like to ask 
your advise again!

I have a miniature blueberry bush  - Vaccinium corymbosum 'Northsky' - on 
our balcony.  I have had it around three/four years and it seems to 
flourish, and each year produces a mass of flowers, but it never fruits. 
It 
is supposed to be self fertile, so not having another plant isn't the 
problem here.  We live in N. Derby****re, the balcony is south facing, so
it 
gets plenty of warmth and sun during spring/summer months.  It is in full 
flower again at the moment but I know they will just fall off and that
will 
be it - no berries!  Any thoughts would be much appreciated!  Thank you!

Tracey