I have a S&W MODEL 649 that has a light hammer strike , it does not fire all the time ! any suggestion that I can do to make it fire correctly. thanks GERALDO

It could be a few things. First did you obtain the gun new? If not someone may have cut a few coils off of the mainspring that pushes the hammer forward. Also check the firing pin for how far it protrudes into the chamber, dirt or debris inside the action binding the pin or anything else. Chances are that if it is used someone changed or shortened the mainspring. All of this can be checked by removing the sideplate. On You Tube Midway USA has several short "HOW TO" videos you can check out to see how to remove the sideplate correctly and go from there. Good Luck.
Chances are a new mainspring will put it back in action as long as it is clean inside.

I have a 642 that did it out of the box, sent it back it came back with new firing pin and cylinder crane, go figure.

I had the same problem with my 642 right out of the box. Sent back to S&W and had it back within a week. Problem solved.