ryandh Trigger pull on SS will be heavier on a setup with good tolerances, too heavy as in if you were struggling to pull to release the hammer means the trigger needs to have more material removed on the rounded corner.
I can't be sure exactly but the first thing that comes to mind is your bolt weight cut may not be in spec if not enough material is removed in the right place, allowing the lever to move earlier than it should when the bolt returns causing the hammer to release early.
(Sorry no good diagram from me for the 9mm bolt weight cut) but I think from your description of your bolt weight cut you are on track maybe just need to go deeper on the grinder, maybe a picture is in order so we can get a visual on your bolt weight.
One last thing is when testing with the upper do you get any kind of bolt binding when charging the bolt to the rear? If you do I recommend filing the part of the lever that contacts the BCG, then polishing, and finally a drop of oil on it.
