The new assembly pin was designed to keep the pin from walking out. It does this by holding the spring much tighter than the ancient one that you lost. Consequently it is much harder to remove. Because it is harder to remove, it is very easy to pull the grip up too far during said removal.
When the grip is pulled up too far, the spring slips out of it's groove in the frame and keeps the grip from going down all the way. Now, the pin will click into the spring, but it is not seating all the way into the frame because the spring holds it out.
So now you have a new pin that really holds, but with the spring popped out the pin can walk out.
You must reseat the spring. After that, whenever you pull the pin, you must also maintain pressure on the grip so it cannot rise up and allow the spring to slip.
Another way to reseat the spring:
1. Unload and remove slide.
2. Mark an X on the left side of the two frame pins.
3. With a 3/32" pin punch. (Sears Companion set--$4) drive out from the RIGHT side only the FRONT frame pin.
4. Pull up the front of the frame while holding in the springs on left and right.
5. Reseat the assembly pin in its frame groove.
6. Push the frame back down and insert the frame pin from the left with the X on the outside.
7. From now on hold pressure to prevent the grip from rising along with the pin.
Packer.