I have two separate thermostats, one on the ground floor and one on the second.

The ground floor thermostat controls the heat on GF and 1st Floor

The 2nd floor thermostat controls the heat on that floor only

Recently when I switch on the GF, the 1st floor heat does not come on. It only comes on if I switch on both thermostats at the same time

I have aired the pump and the rads. Lots of air came out but it is still happening. Pressure reads 1.2bars.

Thanks