You might check brood space - I just had a hive swarm very late in the
honey flow - the flow was so good that the bees drew out 3 deeps of
foundation and filled them with blackberry honey - and then continued to
fill what was left  of the frood space (I could have put a 4th box on!).
 
It might be that they are situated too high in the hive (the queen won't
go down), etc. Make sure the queen has lots of room and good quality
comb to lay in, or they will keep taking off....