What is happening is that the sear/s is now pushing on the trigger plate. What Brother Drew stated is correct in that when putting on the left or right lock plate the trigger/s have to be pushed forward until they don't move and then slide the rest of the lock in place. Same applies to the opposite side. If you look into the hole where the sears go you can see the trigger plate will lower to let the sear slide in over it when you push the trigger forward.
You also have to make sure the lifters are up to allow the long leg of the lock to go into it's fit.