You say “thermostat is in the hallway of second floor”
The hallway used to lose a lot of heat through its ceiling; it does not do so any more, so for a given temperature, it needs a lot less input of heat.
A bedroom also looses less heat through its ceiling then it did before, however it still loses heat through its outside walls. So for a given temperature it needs a little less input of heat.
The thermostat is reducing the input of heat based on the needs of the hallway; the hallways needs have reduced more than the rest of the house.
So you need to turn down water flow to the radiator in the hallway, so it takes longer to heat up. (If you have a hot air system you need to block up its outlet into the hallway.)
This is why it is best to put radiators under windows on outside walls, so the parts of the building with the most heat loss gets the most heat input.
The normal setup in the UK, is to set the thermostat higher then you want, then control each room using TRVs. There are better setups but they cost a lot more.