It could be a few reasons, but my first guess would be that there's a limiting dowel in your magazine tube that's binding somewhere, preventing the shells from being loaded. They may also be a bur inside the tube thats preventing the follower from moving.
Completely field strip you gun down to the stock, receiver and magazine tube. With a hair dryer, heat gun, torch (Don't get it too hot!) warm up the area where the magazine tube threads into the receiver. This should allow you to twist off the mag tube and get to the problem.