I have a 2 zone hot water, gas heating system in my home (1 heater, 2 zones).

When the temprature drops below 35 degrees, hot water does not cycle through my 2nd floor heating zone.

The zones works fine when its above 35, but when its below not hot water circulates thorught the zoen. My 1st floor zone is working fine.

Check the water pressure in the boiler?
Bleed the top zone.
Is this baseboard heat or fan coil?

Baseboard hot water

I did blead the top zone, but I don't know what or how the water pressure can be checked. There is a meter on the boiler that indicates the psi is about 14 psi. There is a red dial that stays at 30 psi.

Did bleeding help?

You may need to increase the pressure temporarly in the system to see if a bit more height/pressure will help. Usually there is a water pressure regulator on the water feed line.