Shrug, pull off the hose and check for obstruction on the elbow of the pipe. Are you getting a loud sucking sound from the overflow? The design doesn't allow air into the pipe except for from the front, the downward flow of the water through the pipe will bring air with it. Might be causing a vacuum effect in the pipe restricting the flow even when finally fully submerged because air cannot escapte properly. If you put a T fitting on the back of the overflow with a hole drilled in the top you may find you get much better results.