I have a 4 frame vertical observation hive in a classroom at the outdoor school where I work. Last year's hive dwindled and eventually died off in March.
I cleaned the hive out, and put three frames of young brood, with workers in it in April (from another hive). They made a new queen, but never really got going.
They replaced queen 1, then replaced queen 2. Finally, their numbers decreased to the point that I removed the remaining bees and started over with a three frame nuc that I had split from another hive.

They did very well for a few weeks, but are now are having problems. The brood appears to be dying while capped. The workers are cleaning the brood out and letting them fall to the bottom of the floor, where they accumulate until I vacuum them out. The brood is all white and don't appear deformed at all.

The hive is inside a building without a/c and it hasn't been cold enough at night for the room temp to drop below 65 (F).

Anyone have any advice for treatment or at least what could be causing this? I have three other conventional hives within 1 mile of this hive and have not had any noticeable decline in any of them.