Older stock (Sprinters, Mk3 carriages) have a simple toilet. Water comes from a tank, flushes a normal ceramic toilet and flushes the waste out of a pipe straight onto the tank. Pro - they are simple to fix and harder to block/break. Con - they dump raw sewage onto the track regardless of where they are (so in stationslevel crossings/foot crossings/pointwork)
New stock has what is effectively a portaloo chemical toilet, hence the fact the scent of newer train toilets is that of a portaloo. So they flush from a tank to what is clearly the plastic bowl of a chemical toilet and the resulting waste is stored in a retention tank under the train. Therefore they block and fill up. In order to empty one they need to be at a depot with the equipment to empty them so if stock is not in a depot overnight and they have a blocked or full toilet then that toilet is staying blocked. It isn't necessarily a difficult job but a job that needs equipment that is not always available.