First off welcome to the forum I think you will find that most members are more than willing to help with any question you may have. Sometimes you will get many opinions that may differ from each other, just remember all are trying to help you solve your problems. Most answers will be based on what we can understand of your problem.
When you can help others please feel free to answer as well. Sometimes someone will have a different experience with the same problem, and can shed some light from a different perspective.
My answer to your problem in this case is based on a problem with a model 36 Marlin that came into my posesion many years ago with a similar problem, and it was resolved with what I have listed below. All of which were the cause of the problem.
First check the loading gate screw to see if it is loose, then give the gun a thorough cleaning. It might just be crud in the locking bolt raceway or the cartridge elevator where it goes through the locking bolt. Hope this helps.