It's likely that there's still an air block in the upper loop. It depends on where you have purge valves, how best to clear that. Are you sure that the pump is getting power? While they are quiet, it is often possible to hear them running if you listen carefully. Is there a zone valve anywhere that is not opening? If so, they usually have a manual open lever. When the power is off, things cool off, the water shrinks, and if there is any leak anywhere, it can suck air into the system.