There are a few abnormalities that will keep the lifter locked.
1) If the bolt/carrier combination is slightly out of battery. This will kick the rear of the bolt stop out and lock the lifter. Have you taken the forearm off to see that your piston is getting fully forward? If it isn't the bolt won't lock into place, the carrier (or slide) won't be fully forward and the lifter will stay locked.
2) If your lifter is bent, trigger group is out-of-spec (or damaged) or you have installed a side-saddle (too tightly) that replaces the trigger pins, the lifter can have issues. Any of these can result in the lifter being out of alignment with the receiver. In most guns, the functional lifter is actually canted a bit away from the shell stop side of the receiver, so that could be an area of concern for your gun.
3) If there is something lodged behind your bolt release/shell stop mechanism. There is a lot of grease in this area from the factory. It will sometimes attract small pieces of material from the manufacturing or shipping process and prevent the full range of motion of the shell stop. You can go further with #4 below to further analyze the problem.
4) If the bolt release/shell stop mechanism is malfunctioning. This is usually because one of the pins is broken or is missing. If you look at your diagram in the back of the manual, there is a clip that keeps the primary shell stop pin in place. Verify that this clip and the pin are both in place. If minor movement of the bolt release button addresses the problem you may have a problem with this primary pin, or possibly the secondary pin that holds the two halves of the mechanism together.
Checking all of this may seem a bit daunting, but you will get to know your gun and likely find a simple problem that is hopefully caused by a piece of debris where it shouldn't be.
William
