Quoted:
Try the disco test very slowly. I have seen one fail when the trigger is pulled slow, but pass every time it was done fast.
Try that to see if the release timing of the hammer from the disco is too advance, hence the trigger front sear not in the correct position to catch the hammer when it's released from the disco. If that is the problem, then either replace the FCG, or being the disco back into tune.
Next try this,
Hold the trigger back, pull the charging handle all the way back, then let the charging handle/B/C fly home while you are still holding the trigger back and no mag in the rifle.
Now release the trigger, and see if you can pull it again. If the disco is not holding, then the hammer is going to be release when the B/C slams home. If the disco held the trigger, then hold the trigger back again (tape if if needed, lock the B/C on the bolt catch, then hit the release to allow the B/C to crash home even hard.
In both cases, the disco should have held the hammer as the trigger was retained, and if need, Replacement FCG, or if the parts are still in good shape, new disco spring and re-tune the disco.