[Mike Huddart]

We have come accross quite a few instances where the locks pull through the retainers and nearly every time it's a valve spring problem. Either wrong springs fitted or incorrect spring height giving insufficient pressure leading to valve bounce. Don't dismiss it as an option, valve bounce will destroy most things, even loosen adjusting nuts like your's.
The worse case we had was on a Cleveland, wrong springs gave only 40 lb seat pressure, by the time the locks had pulled through it had 15 lbs, yes 15 lbs seat pressure.
I am sure yours will check out OK, but I know that's the first thing I would be looking at.
Mike

P.S. that said, we have also had retainers with no heat treat, make sure they're not Chinese !