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....