If the disco will not release the hammer cleanly at trigger release, push the trigger all the way forward and see if this solves the problem. If it does, then the problem is either a bur in the trigger side C slots, or the sides of the disco hole (if the hammer spring is installed correctly).
If pushing the trigger forward does not allow the disco to release the hammer, then check the edges of the hammer and disco sear for edge burs that may need to be removed (stoned).
Also, the FCG should have been CLP lubed before it was installed. If you installed the parts dry, pull them back out, lube them, and reinstall.