Sounds to me that the cam is too short. The cam is that little " heel " of the boot that makes contact with the upper arm of the frizzen spring. The spring should be mounted in such a way that there is enough movement upward after the frizzen is released to open that it stops the frizzen from rebounding back and hitting your frizzen.
Fixes: 1. Get a new frizzen from whoever made the lock. It should be their cost, not yours.
OR, 2. Weld a bit of metal to the bottom of that cam, and file it down to shape.
OR, 3. Put a thin leather washer between the frizzen and the barrel, or bridle, running the pivot screw through it to hold it in place. This will provide enough friction to keep the frizzen from rebounding, but will not interfere with the frizzen opening up.