....in either DA or SA it carries-up and locks in place. But when doing a very slow and deliberate double action, it's HORRIBLY difficult for me to get it to advance and lock and hold -- before dropping the hammer.
Personally I wouldn't worry about it. It carries up and locks in both modes in normal usage, and it's only when you do a "very slow and deliberate double action" that it doesn't immediately lock up, a situation that won't occur in normal use. The cost of sending it back to S&W to make it "perfect" would not be worth it to me. You might be better off to find a local smith that could fix it for much less.