A "blocked" soakaway can mean two things:
In chronic cases, a failed soakaway creates a localized water table. This can lead to rising damp in nearby walls or, more dangerously, ground instability. If the water saturates the soil beneath foundations, it can cause clay heave or subsidence, manifesting as cracks in brickwork.
A professional drain jetter can insert a high-pressure nozzle into the soakaway inlet. This blasts silt off the walls and agitates the bottom, allowing some water to escape. Note: This is a temporary fix for light silt, not for biomat or roots.