This is a gross generalisation but single triggers on conventionally stocked vintage guns are a bit of a nightmare. I have dealt with enough (it didn't take many!) to avoid them like the plague now.
The problem can often be traced to poor restocking or shrinkage/swelling of the wood or simply bad gunsmithing sometime in the gun's past.
Of course they did all work once upon a time and most Boss single triggers still do (subject to the comments above) but they are usually a plumbers nightmare of tiny parts that can easily go out of timing.
Greener used the pendulum design which has been refined by every OU manufacturer for over a century and is now almost totally reliable but the same can not be said of many others.
