The Saiga shotguns need to be broken in like most other guns. Some of them will work with light loads, right out of the box, others will not.
There are several fixes for this. One is to take off the gas block and check the gas holes. Enlarging the gas holes in one way to do it. Another way is to use a 1911 spring on the front part of the upper spring assembly. The front part of the hammer may be impeding the progress of the bolt when it comes back and trys to pick up another shell. The hammer can be removed and ground down so that the bolt touches it only when it needs too.
There is a website, Saiga12.com that is devoted to the Saiga 12 and the common problems. Study up on it an all of your questions will be answered by people that have been there and done that.
forum.Saiga-12.com