Very simple and actually works always. Remove the siphon and clean it. Grab a garden hose and push it about 25 cm. in the drain. Then close WITH DUCK TAPE the drain properly closed. First, turn on the calm water hose and make sure he's not too many leaks in the drain. If this is right then put the valve fully open and rinse only. Just 2/3 minutes rinse. Gone is the blockage. A simple and pretty clean job.