Check the strike angle. The flint is probably hitting at an acute angle instead of a slicing angle. That is a sure flint breaker. Also make sure the frizzen is opening smoothly with no gritty feeling. Polish the spring and foot then grease it. A picture would help.
I have two small Silers and they work very well. I did have a problem with one installed on a pistol with a single trigger. The fly was low and visible such that it interfered with the cock. Simple cure was removing the fly. I agree that the flint must be hitting the frizzen almost straight on instead of at an angle. That can be taken care of very easily by adjusting the flint.
I have tried smaller flints and so far so good. I’ve had a small Siler lock backordered from TOTW for 3=months so having Chambers work on it is out of the picture, I believe makes them for track. I’ll try to get a pic on this weekend.
Lock spring seems strong when compared to other locks, no gritty feel when moving frizzen. 42” swamped barrel, rifle weighs 5 lbs 12 oz empty, that’s why I bought it. Now if I can get the more dependable. Here’s a pic.
Sending it somewhere should be a last resort if you simply cannot solve the problem yourself. You say so far so good with smaller flints. I was going to ask you if it improved any with knapping the flint shorter. Your picture shows a darn near 90° strike at the top of the frizzen, and it should be slightly more angled down than that. It's supposed to be a fast scrape down the frizzen that pushes the frizzen away, not a slam bump that knocks the frizzen over. There's things to do with that flint before you throw your hands up and pay somebody else to deal with it.