Sounds like you are having trouble tensioning the sear spring.
Use the safety to hold the sear while installing the hammer bushing, hammer and disconnector, it a little tricky holding the safety, lining up the hammer pin and making sure the sear spring ends up on the back side of the hammer pin, but it can be done,lol.
Once you have it back in install the MSH (main spring hsg.) and check the function with the upper off, cock the hammer apply safety, drop the safety and hold your finger in front of the hammer to keep it from going to far forward and pull the trigger. Every thing should work before installing the upper.