Saniplus works for WC but not for shower?

When the WC is flushed the Saniplus works fine after a short delay of one or two seconds. The shower is also connected to the Saniplus. The shower tray can be filled to the brim but the Saniplus will not activate. The shower tray brim is higher than the top of the Saniplus. If the WC is then flushed some of the shower water will be drawn off. It requires five flushes of the WC to remove all the water from the shower. There is no blockage in the pipe between the shower and the Saniplus. Is it a problem with a switch inside the Saniplus or a valve where the shower pipe enters the Saniplus?