The spring for the Slide Stop Lever could be installed incorrectly.
Make sure the Slide Stop Lever spring is under the Locking Block Pin. The spring needs to press down on the Slide Stop Lever... If the spring is installed over the Locking Block Pin, it will pull up and lock the slide after every shot.
Remember to put the three pins back in the following order... Locking Block Pin... Trigger Housing Pin... then position the Slide Stop Lever with the spring under the Locking Block Pin... wiggle the Slide Stop Lever while you push the Trigger Pin in... You should hear a "snap" when the Slide Stop Lever spring slips into the slot in the Trigger Pin.
...also... It's best to install the three pins from right hand side of the gun to the left side... Makes it easier to get some of the pins in...
Hope that helps... If not... post back and we'll see if we can help.