When I refilled the coolant in my car. I poured it in the
thermostat hole in block and radiator hose. Put thermostat back in. Started car up and put the leftover in the coolant
reservoir. Sounds like your closed thermostat will not
allow coolant into radiator and block until it opens.
I did not want to risk not having coolant to water pump
so I did it this way. Also put car on a incline just
to try to minimize trapping any air.
Hope this helps,