I would imaging something the size of oregano would be caught with or without the DE in the system. Don't know what kind of valve you have on the filter but if you inspected the grid, fingers, or whatever they're called in the filter, then I would suspect water is bypassing the filter at the valve. Maybe a multi-port? If it is one of these then there is an internal gasket that could be messed up. Any more info on the valve? Model etc?
Al