If it still happens then either you haven't set it up optimally or it's not designed well enough...

Nothing I tried made the Dog Fang work 100%, but it was still an improvement... except for when the chain did make it off the ring - it certainly didn't help when I had to put the chain back.

The difficulty in setting a chain catcher up is that it's virtually impossible to eyeball perfectly and often it's hard to get the chain to jump off intentionally; the best you can do a lot of the time is go and jump off gutters while downshifting.