It could be a number of reasons. I would start with the magazine tube and see if there is something stuck in it. You should be able to see the end of the plunger if you look into the chamber. If you do not, try sliding a stiff piece of string down the magazine tube to see if it goes all the way. Also look and see if the magazine tube is dented or bent. If you do get ammo all the way to the chamber and it does not feed from there you should clean it and make sure it is assembled properly.
Here are exploded views and disassembly instructions.