If it re surfaces then suspect as post 6 TonyL's last comment. The head of the sear engagement screw can foul, the woodwork, as it moves through it's arc. Very hit and miss so relieving the clearance at the trigger slot will sort it.
Laying the rifle upside down with trigger guard removed then operating the trigger shows just how tight the tolerance is.
Just slackening and re tightening the stock screws can cause it to come and go. Look at excessive screw length on this one.