The tumbler looks 'canted' in it's position under the bridle and up against the back of the lock plate.
The rear edge of the tumble where the sear edges are located is up off of the plate surface a small amt.
The front arm or hook that engages the mainspring is down onto and scraping the lock plate surface.
There is a scar mark in the plate from it's movement. I assume the scar has been worn there only when the mainspring pressure has been in play/ in place.
Either bridle needs to be fitted better to hold the tunmbler down more squarely onto the plate,,something you can't always easily accomplish given the as cast qualities of parts and the dimention of the already drilled and reamed holes it runs on.
But the engagement of that arm of the tumbler may itself be at a slight angle and not square withnthe mainspring hook. Or the spring hook itself may be finished at an angle as well. either or both together when engaged with the powerful spring tension imposed onto the tumbler can push the entire tumbler one way or the other if the pivot holes and axels are not fitted well.
In this case I think the tumbler is being pushes in towards the lock plate at the juncture of the mainspring hook and the tumbler arm.
As the lock is cocked the rotational motion of the tumbler may be being stalled just before the sear can drop into the full cock notch as that arm digs into the lock plate.
Remove the mainspring and everything works fine w/ lots of play in the parts and the arm of the tumbler free to slide above the plate surface.
I'd examine that carefully and if a problem I'd file some relief to the back edge of the end of the arm of the tumbler and remove any sharp edges around the part(s).
Check the plate with the mainspring removed also and make sure the spring,especially the hook, isn't doing the same thing of scraping against the plate especially at it's high point of compression.
The lock plates are often unevenly polished 'flat' and especially under the edge of the bolster there may be a slight high spot that the spring can run into and stall as well.
