A few things you can try. Simplest is to keep the water level high, let the weight and movement of water keep pressure on the clog and slowly work it through. I've done this for a clogged sink as well. If the water level is dropping a little, that's good, there's a small hole somewhere and continued water moving will erode it bigger until it gives way.

Quicker way is to find something long, flexible, and probably disposable that you can use to get around the bend and push and poke away at the clogged area. You just have to get it around the curve, and it won't even take much of a hole to get water to move and help.