Low Water Pressure in Heating Line

[CP]

I have a two-stage gas fired boiler with an indirect hot water tank.
The hydronic circulator is in between the hot water Tank and the
heating system and is in the back side of the two stage boiler.
There
are drain valves above gate valves on the front and back of the
heating system (supply and return). The normal psi for the heating
system, under load, should be 20 psi.

One of the apt unit's heating line is always cold (highest and
furthest away from heating system). All other units heating lines
seem to be fine.

I have repeatedly attempted to drain the air pockets from the
problematic heating line by opening the bolier drain valve and
closing
the gate valve after it and have witnessed low water pressure. When
I
bump up the pressure in the heating system to 30 psi or so, I begin
to
get more water flow, albeit cold.

What could be the problem? How can I diagnose it?

CP