Does that explain why switching to the main drain has the same problem? I guess that with 30' to the pump that valve is near the pool and the gunk got past it. May be difficult to do but can you hook up the pump backwards and try and blow the obstruction out with the pressure and volume the pump can deliver? A garden hose obviously won't hack it. Higher pressure but low volume and you can't develop enough force to knock the obstruction loose.
Al