Make sure the area where the barrel block was is milled out and flush with the barrel channel and needs to be clear of any obstructions and contoured to the existing channel shape. This includes the locking block screws, which sometimes requires filing them down inside of the barrel channel to prevent making contact with the recoil spring when they are fully tightened and instead could be backed out just a bit to accomplish this as well.
There should also be a small notch or divot dead center on top of the rear wall of material that connects the two rear rails. Make sure this area is smooth and clear of burs. In addition, where the trigger bar makes hard contact with the frame at the rear where it curves, a slight filing away of the polymer material will allow the trigger bar to smoothly travel rearward when cycling. The nub on the trigger bar that makes contact with the safety plunger underneath the slide can also be slightly polished and cleared of any burs or excess material that may interfere with the slide properly functioning as it cycles.
The trigger mechanism can be shimmed by placing a cut credit card or similarly thick piece of material in front of it, between the trigger mechanism and the thin wall of material. This is the typical culprit of any difficulties in releasing the trigger.