Like Retired W4 stated, someone has most likely got the swapped the screws around. On most S&W models of revolver, there are three screws that are on the sideplate (older models have four). These three screws can be interchanged. However, the front screw is fitted. It is what holds the yoke into the frame. So, if you put one of those other screws in its place, the yoke will either not open at all, or it will be very sticky.