Remove the downspout from the drain pipe. Insert a garden hose and turn water on full. If water backs up quickly, the clog is near. For tough clogs, use a plumbing snake or sewer jetter hose.
Insert into the pipe; it expands to seal and blasts high-pressure water forward. Warning: PVC pipes can burst if pressure is too high – use with care. underground gutter drain clogged
Alternate flushing from the downspout and from the outlet end. Back-and-forth water pressure often breaks loose stubborn debris. Remove the downspout from the drain pipe