I realize that this is an older thread, but I encountered it while searching for information on Cracked frames on shields.
Mine cracked today while at the range, or at least, it had a number of malfunctions today. While researching the malfunctions, I came across a different thread on this forum, describing the same problems that I was having. The poster found a crack between the trigger and the magazine release button. I checked my Shield .40, and there was the same crack . It was apparently allowing the frame to flex enough to allow the catch on the slide release to push the magazine over against the trigger draw bar, and prevent engagement with the sear.
The mythical frame crack that everyone in this thread agrees doesn't exist...exists. I'll be calling S&W tomorrow.
How old is your Shield? Recent production or a 2012, etc?