I got a non-Buff five foot octa off of eBay, and it fell off, but only fell off on ONE of my Einsteins. Buff customer service basically said "too bad so sad" but in a slightly nicer way. I sort of understand, it's not their product that's falling off, they have no idea of the quality.
Anyway, I solved the problem with the rubber band trick. Specifically, I used Bongo Ties and on the upward pointing lever I wrapped the band around the umbrella screw, and for the downward pointing lever I wrapped the band around the knob that secures the light to the stand. With that configuration the five foot octa stays on even with moderate jostling of the light. I normally let these sort of things bug me, but it's a cheap and very quick fix...so I'm simply not going to worry about it any more. Yes, I'm irritated that the design isn't better, but I now know to make sure the levers are as engaged as they can be and to use my Bongo Ties. All done.