The most common problem is that the end of the screw is rounded over instead of flat. So when you clamp it down, it is not making full contact with the blade. Sometimes it works to just turn the screw a quarter turn, other times you just need to file the end of the screw with a fine file. Does not take much to fix it. The other problem is that the screw head may not be sticking out far enough into the clamp face.