I have a beloved S&W Stainless 645, but it doesn't get much use. I had a chance to take it to the range the other day and noticed that while it was very clean and properly lubricated, it had a problem reseating itself occasionally between rounds. It fed just fine and I could easily reset the remaining 1/8th inch with slight pressure, but that's clearly not the way its supposed to be.

I could not find any evidence of burrs or other damage which would cause it to hang up like that. Weak spring? What should I look for?