+1 on what Rusty says, "that is an odd problem".
My thoughts are maybe the crosspin can mis-align and cause a drag on the firing pin. A small change in lock time can correspond with shooter mechanics leaving undesirable results.
Just maybe!
Can you change just the bolt body? Might be worth a try. Haven't bought one lately but the last ones I bought were from SSS a few years ago. Give Lisa a call, I hope they still stock them.
Randy