Hmmm... lots of interest...
Okay, to save time I'll answer the last three posts in one shot.
1. Do you have another speed ring to try it with?
Yes, in fact, several. They all look EXACTLY like the one on the big box, so I'm inclined to believe it's an issue with the fingers/springs rather than the ring.
2. By slipping off, do you mean that the softbox just falls off to the floor or does it just not stay where you put it when you rotate it to other than vertical?
The box falls off on the floor. I might mention that the box is the older LightWorks box AB sold, not the current "folding" version. It's relative heavy (okay, so maybe 4 to 6 pounds... I haven't put it on the bathroom scales yet!)
3. Maybe it's put together backwards or something. Maybe your light is damaged too.
Put together backwards? Don't think so... It works just fine with the medium softbox, it just seems the extra weight/torque of the big box is too much for the fingers to deal with.
Damaged light? Again, don't think so as everything WORKS just fine, except the fingers just don't hold the box on securely. After a bit, crash, and the box is on the floor unless it's near perfectly vertical (meaning little force pulling it forward).
Elvis has left the building...