Soakaways fail because they silt up, because the surrounding soil has become less permeable, or because they were undersized for the roof area draining into them. Clay subsoil, common across the Weald, shortens soakaway life considerably.
The test is simple: watch it during heavy rain. Water that stands rather than disperses within a reasonable time indicates failure.
Repair options range from jetting and cleaning the inlet pipework to excavating and installing a modern crate soakaway sized properly for the catchment area.