Also, I will say that the original instructions weren't super clear on first read through. The last course I taught there were 2 new techs who weren't familiar with the old style of Britax seats or any lock-off, so they followed the manual step by step and still missed opening the 2nd piece. I read it twice before getting the wording when I realized there was no way the lock-off could possibly close that way over the webbing and it just didn't make sense to me.
There's a leaflet in the new seats with a better picture and clearer instructions on use of the lock-off.
IMO, the design is counter-intuitive and not user friendly at first glance. All the other lock-offs I've seen are obvious when they're 2 pieces. Once you know these are 2 pieces it's easy to tell, but the spot where you reach in to open the 2nd piece isn't really that big, so it's easy to miss.
So I wouldn't beat yourself up over it. Call it a learning experience, and apply the lesson for future reference. :thumbsup:
FWIW, I'm sure that there will be multiple lurkers who read this thread and all the sudden realize what their problem is, so others will learn from you sharing your mistake and that's something to feel good about.