Queen on the downward trend perhaps already dead ( hornet attack ?? , insecticide or disease. The hive is confused , stops defending itself , so the hive gets robbed by a stronger hive . Hope it was not disease or insecticide .
It is a common thing this time of the year for weaker hives to be overcome and robbed by stronger hives or feral nests.
The hives are also killing off the drones for winter survival as the queen should have enough sperm in her to lay through winter into swarming season next year. Did you have any frames of drone only brood comb? that would also make for big reductions in bee numbers . I never liked the idea of providing more homes for Varroa than the bees could sensibly cope with nor the hive having to feed more drones than nature intended if the Varroa drones hatched due to my neglect with so never took the separate comb route .
The old queen may also be in the fortunate position for you of superseding naturally . When you looked the new queen may have been out on a mating flight . I can't for the life of me remember how many days it takes for a newly mated queen to start laying eggs . So long as any eggs you have been seeing are upright inthe bottom of the cells there should be nothing to worry about
To reduce the robbing problem & also a good idea in any case , you could try making wooden hive entrance blocks and make them so that there is only a two width single height bee space to help hives get over this theft/attack problem .
This time of the year also take the precaution of securing a decent mouse guard over the front of the hives leaving the reduced access gap accessible .
.
I would have normally finished feeding my 25 to 50 over wintering hives in a normal year by now. But this year would perhaps do it for another 14 days but make it a thicker sugar syrup with less water for the bees to evaporate away .
Give it another few days and perhaps unite two weak hives , brood box to brood box built up as a single hive with the hive block & mouse guard fitted then , feed it like mad till the bees stop taking syrup or till a week on average before your first frost.
Unite the hives with a sheet of newspaper to separate them and poke a few 1/8 inch ( 3 mm ) holes in the paper just before you unite the hives, four days later remove the paper set on a queen excluder on the bottom hive and place the top section less than a metre away, on a temporary base with a lid over it , the bees will fly to the hive with queen in it .
Once you have determined where the queen is . Use a handful of long dried grass or a new long feather to brush bees off the combs of the emptying hive , don't use a disease spreading bee brush under any circumstances , burn it if you have such an implement . Only use that dried grass/ feather for that brood box and only use it once to keep disease spread to a minimum.
Add any brood or eggs from the emptying hive to the queen right hive but cluster them to only one side of any existing laid frames. In other words don't split the brood nest in the combs of the queen right hive .
One other big reducer of worker bee numbers is if they have been working heather in warm windy weather ( read strong nectar flows ) . A normal six week old worker bee at death gets trashed at about three weeks from horrific over work and wing damage .