Or a MIM part failure. My 686 developed a problem with the cylinder not wanting to turn double or single action. Sent it back to S&W. They sent it back saying they changed the hammer, trigger, Hand and the ejector rod.
I have a 7 shot 686 with the lock. I removed the lock. After several range trips the cylinder hand start to fail. It went back to S&W. They replaced the hand and the rest of the MIM parts. They also replaced the lock.
They had it back to me in 5 business days. No charge. I left the lock in this...
I had kind of the same problem a couple years ago with a semi new seldom fired 686. I suspected the hand was bad. I sent it back to Smith.
A week later I got it back from them. They replaced the hand and the rest of the mim parts. Been good ever since.
John!