If you have installed a manual vent valve at a possible air trap in a riser to vent air from the tubing when the zone valves are closed – then found no air coming out look for something else.
There may be an expansion tank or bladder type device in the pipe somewher besides the expansion tank near the boiler. There needs to be a place that a large slug of water can follow an air pocket to slam against an elbow.
Also the system pressure may be below that needed to keep water pressure at the top of the loop. If there is an automatic air bleed there, air can enter the upper pipe while the zone valve is closed to permit the slamming and then the air can exit while the pump pressure forces air out.