As Daryl says check the flint.
I fit frizzens to pan with files and scrapers. A small scraper made of O1 and drawn to a straw temper will cut soft parts easily and allow removing metal that cannot be done with a file without removing the surrounding area.
The problem may be that the pivot hole in the frizzen is a little too high. This will make it difficult to impossible to get a good fit.
If this is the case, the frizzen gaps at the pivot end, if you buy a new frizzen it can be fitted then the hole drilled with the frizzen clamped in place. Having a soft frizzen allows it to be files and scraped as well. The hardened frizzen can be annealed.
I use blue or black magic markers to find contact between the parts.
Most people who assemble locks do not ream the various holes for a tight fit. Simply drilling with a clearance drill makes for a sloppy lock. Especially the frizzen.
The other option, assuming the lock has not been engraved or much modified, is to replace it with one that has a better frizzen fit.
Dan