...(bullet and case, no powder or primer)...But as soon as I secure the spring inside the mag tube using a toothpick,...
The "no primer" might create an additional function testing issue, the nose of the bullet may enter the primer pocket.
The magazine spring should be inside the magazine follower. Any chance the end of the spring is caught between the mag tube and mag follower?...a clipped end maybe?
Is the follower bent or out of round?