Hi Robert,
I have replaced several digitizer/LCD-units and sometimes, but not always, I had the same problem.
I always check out if there is something stuck in the clamp holder of the metal clamp and if it really "snaps in". But it seems, that in your case, everything is clean and works.
I recently "switched" to super glue to avoid that metal clamp/bracket to get loose or to move, even slightly. It seems to me that the adhesive that I used was sometimes not strong enough to hold down the digitizer/LCD-unit permanently. With super glue, however, the problem seems to be solved. I would Just put a few dots on the back of the metal bracket and let it dry.
But there is one disadvantage: Once you use super glue, the metal plate can not be removed again in case of a (later) replacement of the digitizer/LCD-unit.
I hope I could help.
Steffen