PVC or metal downpipes can sometimes accumulate sludge along the sides. As you are working with the hose or snake, gently tap the sides of the downpipe with a rubber mallet or the handle of a screwdriver. The vibrations can help shake loose sludge and speed up the flow.
Fortunately, unclogging a downpipe is a manageable DIY task for most homeowners. This guide walks you through the steps to identify, clear, and prevent downpipe blockages. how to unblock downpipes