OK, I now have a better idea of what you are say, when you fire it the trigger doesn't reset. Now, you said it did this BEFORE you did the SP trigger upgrade, if I have that correct.
I wish I had this thing in front of me, I think I have some ideas of what MIGHT cause it. Its far easier to describe when the parts are in your hands. Now, seeing as you got it used this problem may have come with the rifle. There is not a lot of parts involved with most triggers and this one is no different. My question here is whether the previous owner might have modified something a bit "too much" and pushed this thing over the line one some minute adjustment.
There use to be a website that had a good picture thread on taking this group apart. If I had more time I could do a new one, but other projects are higher on the list. For now what I can tell you is that the sear catches the hammer when it comes back after a round is fired. This sear holds the hammer back until you let go the trigger. Now, if you pull the "upper" off the rifle you can manually operate the trigger and observe the parts doing their job. BUT be careful. You need to control the movement of the hammer by restricting it with your thumb.
First, take a look at the trigger transfer bar that goes from the trigger back to the center of the hammer. It has to be in the proper position during various times to make the sear work right and to allow the hammer to function. Notice that the rear of that bar, just before it bends over to enter the hammer it has a "bump" on the top. This bump has to align with a cut out in the bolt to allow this trigger bar to rise up. There is a spring that should be pushing upward on this bar to try to maintain it in the upward position. This is a "Closed Bolt" safety to allow the trigger to only work when the bolt is fully in battery.
Next, as the bolt travels reward the bottom of the bolt SHOULD push downward on this bump enough to allow the trigger transfer bar to engage the sear that holds the hammer back until you release the trigger. (if that bump was not tall enough that might prevent the sear from latching). Also, hidden inside that hammer is a little bar that works with the transfer to lock the hammer back. It has to be free to move also. Study how the hammer locks and you can see just how this little guy does his job. You can operate all of this while the lower is off the rifle and see if these pieces work the way they should. Its not rocket science, once you see what is suppose to happen you just have to figure out which piece is worn, bent, or not adjusted right. You may have to take the hammer out and look inside to see if the parts are all OK, not rounded off or anything. There again, if someone has tried to polish these doing a DIY trigger job they might have gotten carried away.
IF, and that is a big IF, the ammo is powerful enough to send the bolt fully rearward then the problem is likely in these small parts. Again, not rocket science, just a few metal bars interacting. If I recall the SP trigger takes some careful adjusting. But, if you are confident that you have followed all their instructions then the rest you should be able to work through.
Good luck with this. Again, just sit and study it and watch each and ever part until you can understand what each one does. Then it should be clear which piece is to blame. Of course, if you had a working one right next to you it might go easier as you have something to compare it to.
Now, if you need any more info that that you can send me the lower with a blank check and I can fix it for you....
just kidding.....