While I didn't think it would help, I flushed the mechanism, and the water was as clear coming out as going in.
The action is and was smooth when off the bike. I watched closely as I manipulated the lever and saw that the bar raised very slightly to its highest level just before it dropped down slightly and locked in place. Removing the bag would cause the same slight rise before the bar is unlocked.
The slot where the bar locks into has a rubber surface that may have hardened over time. In any case, the problem seemed to be that the bar was too long so I used my mini hand grinder with a sand paper roll tip. I went slowly, just removing a little of the top of the bar, being careful to keep the top of the bar level, and checking the fit on the bike as I went along. After the first fitting it was clear I was on the right path.
My bags now lock on and off with just a nice reassuring click instead of the previous harsh snap. I'm guessing those who did not have the problem got a set of bags with bars that were just a bit shorter than those who have the snap on closing and opening.
Very little material was removed. I would suggest the fix for anyone who feels the pressure required is excessive on their bags,