The fall out trigger uses two set screws to hold the trigger pack tight to the receiver/pins.  So before you install/remove the pack, you have to make sure that the set screws are backed upwards, then one the pack is installed, the the two set screws are tighten up to secure the pack tightly in the receiver against the pins.


So on that note, if you have not backed the set screws off, could be the problem at hand.

Short of that, then check the pack against the side of the receiver to confirm that the receiver pin channels are correctly spaced, and then check the pack inside the receiver to make sure that it has the room needed for the pack to be installed as well.