If water still won’t flow, feed a plumber’s snake (drain auger) down from the top. Twist and push until you feel the blockage break up. Flush again with the hose.
For severe blockages, unscrew or unclip the downpipe sections. Clear each piece over a bin. Reattach securely. how to clear blocked gutter downpipe
Ensure it is on solid, level ground with a 4:1 angle (1 foot out for every 4 feet up). If water still won’t flow, feed a plumber’s