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It seems that the back of the trigger is not dropping down enough to allow the safety to function properly. There are three possible problems causing this.
First is that the FCG pocket needs to be deepened at that point to allow the back of the trigger drop further down.
Second is that the hole for the trigger in the bottom of the FCG needs to be lengthened at the front to allow the trigger to drop further down.
Third, when drilling and tapping the hole for the pistol grip screw, it caused a burr on the floor of the FCG pocket which is not allowing the back of the trigger to drop,
or the pistol grip screw is protruding though the hole in the bottom of the FCG pocket and not allowing the trigger to drop completely. You can test for this by simply unscrewing the pistol grip screw a couple of turns and see if that fixes the problem.