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It sounds like the fan connection is blocked or the venturi is damaged.
Will it work with only the positive pressure tube attached?
It sounds like the venturi is not creating enough pressure or the flue is partially blocked or the flue seals are failing.
More likely to be the first point.
Does it work any better with only one tube connected?
You may find they are both prducing positive pressure
Perhaps the boiler does not know the pump is running.
Does your model have a ch flow switch?