'93 Voyager coolant recovery system problem

Hi My '93 Plymouth Voyager 3.0L has a problem. The coolant recovery tank accepts the hot 'excess' antifreeze when needed, but as the radiator cools, no antifreeze returns to the radiator. No coolant is really lost, it just is replaced by air in the radiator neck. The radiator cap is new (the old one only kept 10 lbs pressure). The recovery tube is not plugged. The cooling system, when tested, drops pressure from 16 lbs to 12 lbs in a few minutes. No obvious coolant puddles under the van. Whta do you think is needing to be fixed? Thanks, Walt