1st stage hitting back of head

[freedivernd]

I can't be the only one that has had this problem... but what was the cause? what's the cure. I am thinking it might have been that the b.c. was a bit too large and it allowed the tank valve to tip down against my head rather than holding the tank/b.c. tight against my lower back. I don't think it was that I was so inverted that I had to tilt my head so far back that I was hitting the 1st stage. I had my top tank strap set right at the ogive of the tank, in fact, if the strap had been any higher it wouldn't have been on the tank at all. Thoughts? This has happened to me before, but also, I have had dives that it doesn't happen.. They are all rented b.c.'s
Fred