5 zones, 1 with major issue. HELP

creativeinstalla

I am the GC on a job we installed 2 years ago.  5 zones.  1 is turbo max, 1 controls lots of radiant, 1 basement baseboard (old existing zone), 1 st floor, mostly new baseboards, and 2nd floor - we added 1 room on to loop.  All were working fine except the 2nd floor.  the pump kept loosing prime.  We and our lumber have been back a few times.  installed a Spirovent on that zone immediately after pump (all pumps are pushing) ,  Installed flow checks on both sides of the loop (after spirovent and on the return before it ties into manifold)  this is Zone 1 on the header.  Every time we went when customer called zone was calling and pump was on.  No water circulating.     Today - I just bled out about 1 -2  gallons of water and then it starts to fully circulate.  We are all at a loss at what is causing this.  the boiler was running at 20LBS when hot.  i pushed up to 25 today.  house is 2 story with boiler in basement.   We 2  #30 expansion tanks.  Pressure relief valve has never blown off yet.   Any ideas on why this zone keep s getting stuck.  Have changed pump 3X and it is probably not defective,  Have a 010 Taco on there now.  Please help!