Unless the face of the carrier, is tight against the face of the barrel extension, FP is being blocked by the back of the carrier, since the bolt as not rotated enough for the tail of the bolt to protrude out the back side carrier gas block channel.

To see this, pull the B/C flip it upside down, look at the stop collar of the FP, then cam the bolt back and forwards.  What you will see, that is when the bolt is unlocked/forward in the carrier, it tail is drain inwards into the back of channel, a until the tail off the bolt can protrude so the stop collar on the FP can seat up against the back of the bolt tail, FP will not be able to protrude out the face of the bolt.

So face of carrier not touching the back of the barrel extension at lock up, when the hammer comes down to strike the firing pin, firing pin stop collar maxes out of back of carrier, drives the carrier forward to lock it up the last of the way, by the time the FP can kiss the back of the bolt tail, not enough FP energy to set the primer off.

So could be ammo, or could be the Larue Extractor Kit, since it adding so much tension to the extractor, the extractor is not able to spring outwards to climb over the case rim right before the bolt  starts to cam close, and preventing complete lock up.

Also, pull the bolt off the carrier and dry fit the carrier with with key to the upper.  You want to make sure that the carrier is not binding in the upper receiver, and the last inch of travel forward as the gas tube enters the key, the gas tube is correctly aligned to prevent binding there as well.  If gas tube has to be tweaked to align it with the carrier key, tube should be bent over the center of the barrel.

Another item, and when was the chamber of the rig chamber brush scrubbed by hand with CLP.  The last of chamber cut can build up with crap over time if not chamber brush scrubbed out ,and could be just enough to not allow the case to seat in the chamber cleaner as well.

As for hammer spring, confirm the coil wind to straight leg section is at the bottom of the spring, and the spring leg is on top of the trigger pin.  Hell, with the rig 20 years old, may be time to just pull the FCG out of it to give it a good CLP cleaning.  The hammer and trigger channels can build up with crap, dries out, and this causing enough binding on say the hammer in rotation on the pin to slow it down enough even if the B/C is correctly locked up in the barrel extension, hammer is not striking the FP hard enough to set the primer off.