A venturi circuit operates by having sufficient gpm through the resistances in the tees of the main to drive some water to the radiation. IF the main venturi tees are large for small flows, they can become undependable, as the resistance in the mains are marginal to drive the radiation loops from them. This can be corrected by adding a balancing valve between the risers so flow can be adjusted to the radiation above (it is the function of the venturi tee to do that; but if it can't, due to lack of planning the flow, then the balancing valve becomes the main control.) As you don't give gpm flows for that loop and btuh requirements for the radiation above, it is likely you don't have the knowledge to properly size the main and tees for that loop, as the installer didn't have.
Also, venturi circuits become air traps, as there is minimal force to move air downward to the main to be purged at air scoops. Multiple zones have many such air traps, but venturi circuits moreso. Air control and elimination requires care.http://www.heatpro.us/1pipehwtree/documents/7F98E674CE9051033975E463F8FA6E8E5EF07CF2.html