Probrably the locking lugs. Closely the barrel into the action very smartly,
cock the hammer, hold it back with the thumb and pull the trigger while watching inside between the hammer and frame. if the small sliding block does not drop down, the barrel lugs are not going into the frame far enough. This can be cured by delicatley stoning off the top of the lug so it will enter the frame far enough. or returning it to TC for refitting. This is a common problem and easily solved.
Good Luck,
Wally