This is a suprisingly common issue, and one I had myself. You may think you put the rag joint onto the box correctly, but it's 90 degrees out. The flat on the box shaft has to match the flat inside the rag joint - it's not the gap where you tighten it, but if you look closer there is a part of the round section that isn't splined.
Here's a pic showing where I'd got it wrong. It the top of the joint, what looks like splines are grease lines where I'd put it in wrong. There are a couple of gaps showing where there aren't splines.