To save on running costs, many of the circulator pumps installed don't have enough head to pump the lines full of water if there is air blocking them. If they are full of water, they'll move the water fine. To get the air out of my highest loop, I had to improvise a means of temproarily supplying higher pressrue water. Sometimes, you can open one of the drain lines, and manually open the makeup feed line (isolate the expansion tank to prevent damaging it first) to get more flow under pressure to help purge the lines. My only practical experience with this is on my own system, so can't say the best way to do it.