SLEEPING!
Well ok, nobody really knows for sure, but there are several things that they could be up to.
1. cleaning the cell, this can take some time.
2. feeding brood, doesn't take that long.
3. moving eggs, there is some debate over egg moving but I think they do.
4. warming the cells, when ever nessary they will put their bodys in a cell to help warm the cells around it.
5. sleeping, No one really know if bees sleep (at least the way we do) but they are known to sit for some time without moving. They will do this ether standing...wherever, or in a cell.
6. Bees will also enter a cell in times of high stress, like when the colony runs out of food. Hives that have starved, will be found with a large number of them inside the cells.
BB