If I understand, you're having a problem inserting shells into the magazine tube quickly?
First step is to fully disassemble the magazine tube assembly.
Clean the inside of the tube with a large diameter tube brush or a large rag with a shotgun cleaning rod.
Inspect the tube for dents, rust, caked on fouling, plastic shell residue, or old lubricant.
Inspect the rear of the tube where it enters the receiver for damage.
Inspect the shell releases in the receiver for damage. "Usually" if the gun is functioning the releases are good.
Inspect the magazine spring for kinks, bends, broken ends, rust, spring set, or other problems. If in doubt, replace the spring, they're cheap.
Inspect/replace the magazine follower.
Some of the thin plastic factory followers expand over time and can cause problems.
Good replacements are another factory follower, of a Wilson/Scattergun Tech bright green follower.