l have just put a dishwasher in and the drain hose as well as the washine machine waste pipe are connected to a downpipe with auxiliary connections for each appliance,this downpipe is just before the u bend under the sink.The problem is when the dishwasher has been through it,s cycle and finished there is still water at the bottom, l have checked the filter and this is not blocked.What could cause this,the drains are fine and have no blockage.