In reply to Will Hempstead:
As you were trying a VII I sort of feel bad even asking this, but do you have your crampons set really tight on your boots - i.e. its really quite hard to level the back bail up into place (perhaps even needing to pull on the strap and definitely risking your fingers if they are in the way when it snaps into place!)? I can't remember the last time I lost a step in crampon and I climb quite a lot in winter conditions even now, and weekly or more a few years ago, normally with step ins, and on all sorts of different boots over the last 25 years+. I watch some people flick the heel lever into place with one finger - virtually no effort at all then hear stories of people's crampons coming off. These things might be connected!
If your's go on really strongly and really snap close, then I bet it was just the weird situation you describe - I imagine on a hard route you are doing everything with power, jamming your foot in, pulling it out to get it on another hold and so on. Perhaps just bad luck?
I should add, I've used BD, CT, Grivel, DMM, Charlet, Salewa and Elderid crampons over the years - but never Petzl, so I suppose their toe bails might just be no good!