Best guess, you forgot to install the pistol grip washer, or the distance on the new grip bolt channel is shorter than the factory grip, and now you have the tip of the pistol grip screw protruding up into the bottom of the receiver void under the back of the trigger, which is not allowing back of the trigger to drop down to it's normal at rest position.  

Check the grip screw to make sure you install the washer, or if needed back out the grip bolt a few winds and see if this solves the problem.  If backing the bolt out solves the problem, then pull the bolt and shorting it by a few threads.