>I found that some of the hives had killed their new
> queen and are queenless.  I doubt that I would have had any problem
> earlier, but the bees are in intense robbing mode.
 
There is the answer.  When robbing is on, bees get very choosey about
their near neighbours.
 
> I had killed the queens and left the hives queenless for a day.
 
A queenless period doesn't help when replacing a laying queen on
comb  with a laying queen on comb.  In fact, it may have contributed
to the problem.
 
If queens are switched without disturbance during a flow, there is
seldom a rejection, but to dequeen, and allow the bees to become
queenless , then requeen -- especially during a robbing period is
asking for trouble.
 
> Has anyone tried spraying the bees with light syrup?
 
During intense robbing?  Not a good idea.
 
The best plan is to choose a time when the bees are calm for
requeening.
 
 
 
At such times, hives entrances should be reduced -- if possible, and
nothing dione that might give any hive an advantage over another.
Regards
 
Allen
 
W. Allen Dick, Beekeeper                                         VE6CFK
RR#1, Swalwell, Alberta  Canada T0M 1Y0
Internet:[log in to unmask] & [log in to unmask]
Honey. Bees, & Art <http://www.internode.net/~allend/>