Can you share any more particulars like the type of lock, the width of the frizzen, the dimensions of the flints you're using, bevel up vs. bevel down, etc, etc?
For example:
If a flint hits too straight on, too square into the frizzen they can break...bevel up / down can change this angle;
If a flint it too wide it's outside edge can be coming down on the edge of the pan and breaking;