The basics, including making sure that all the parts and pins are CLP installed before you install the parts, and Disco spring is installed large coil down into the trigger slot with a set of pliers to force it into place.

Also, Make sure that width on trigger channel width is not too wide for the receiver cavity width, and check the J spring in the hammer, to make sure the hammer center channel is peened correctly to retain it, with the J spring not falling out out the bottom of the hammer that case cause the trigger to be blocked back.

If trigger has set screws to adjust over travel and take up, make sure that over travel is adjusted with hammer unclocked, since can over tighten that one, to the point that the bottom of hammer lob will be blocked by front of trigger.  As for take up, selector on safe, and adjust to that.

As for blown spent primer hiding under FCG, hate to say it, but FCG has to be pulled to check for pieces of it.  
Plus, with FCG pulled again shortly after rebuild, can do a quick clean up of the  pin channels for the parkerizing that has worn off to clean that up, that will make the trigger more gritty feeling during the pull.