Some questions, is this one zone or multiple?
If it is multiple zones are there multiple circulator pumps or zone valves?
Any leaks anywhere? If so you may need to add some make up water. Is there an auto-fill valve and if so is the water feed valve open or closed?
Usually you need to push water up the supply side and purge at the return.
Come back to us with more info and pics if possible so we can better help.