The problem with some hammers having problems is not always directly related to the hammer it's self/alone, but also to the geometry of the hammer as well (distance from the center line of the pin hole to the front edge of the sear).
To keep is short, the FCG must spec as a unit to work in any given rifle, and when installing either a new part or a full FCG, there are times that you will need to re-time or mod a part of the FCG to get the complete working FCG into working specifications.
Since this disclaimer is always present with every single part that you buy,
"Should be installed by a competent gunsmith",
Some fitting/tuning may be expected from even the best of suppliers, especially when the complete rifle may be made up of sourced parts by many different manufactures.
On the bright side, this is more than a handful of members here that can walk anyone threw the simplest of needed modifications if problems present themselves when a part/combo of parts need to be fitted to bring the parts as a whole into working order.
Note: It’s either fit the parts, or you better have bins of them to start swapping until you find the combo’s that will work in the rifle.