radiant no flow

i've got a radiant system with four seperate zoned manifolds. each manifold has a 008, with a honeywell thermostatic manual mixing valve. it has been in operation for 8-10 yrs. the piping off the boiler is a supply out to manifolds and a seperate return back from the manifolds to boiler. there is a hot by-pass from supply to return @ boiler to mantain 135' at return. the bypass is thermostatic with a mixing valve and 007. ive purge air from system, but i can't get flow through manifolds out to system and back to boiler. i can send pictures if you need

jim