I don't think it is a pressure issue, and (I'm not absolutely sure), I don't think you'll need that much pressure. The pressure is to ensure in the boiler you maintain enough pressure to keep it from flashing to steam. The water outlet won't (or shouldn't) be hot enough where a small pressure difference could allow that to happen outside of the boiler. It could easily be air in the system or head for the pump that is insufficient to get it to flow there. You may also need to adjust the balance of the zones to obtain the desired flow.