With the trigger frame off the gun, and the sear not in contact and under tension from the hammer/striker, the sear will not move when you pull the trigger.
Here is a video I made for another member, the spring could be missing or broken on the sear.
Or, there could be a problem with the internals. When was the last time it was taken apart and everything was given a good lubing? Are their any upgrades? Spring Kits? Regulators? Valves?