May be a bit too obvious but have you checked the flow and return valves on the boiler are BOTH open to the system. Also check the large round slotted screw head in the centre of the pump to vent any air from the pump. Just crack it open and expect a little water discharge when you do.
There should be an auto vent located, possibly behind the pump. Check that the little dust cap like plastic cover on top is slack to enable the auto vent to operate during pump operation. These are basic observasions any further intrusions into the boiler may invalidate your warranty. If the problem continues contact your supplier.
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