I had that issue a couple years ago-- different gun but same symptoms, the magazine was failing to feed. Took my magazine apart and found that the follower was gunked up. The piece I'm talking about is the metal or plastic piece that you see when your mag is empty. It's a little cap shaped thing that sits between your shells and your magazine spring. For example, mine is safety orange plastic. There was just enough powder gunk on that plastic that it was catching in the mag tube from time to time.

Mostly I had your problem, but once, I loaded the gun up, took a shot, second shell fed just find and chambered, lifted the gun and I heard the noise of something sliding. The follower had jammed down and I had a shell floating in my mag tube because it didn't spring up and push the shell all the way to the "ready to feed" position.

Anyway, I hit mine with some hoppes but it was stubborn so I ended up lightly scraping all the edges of the follower, cleaning good, and I've been good to go ever since. If you have problems filling the magazine or feeding from it, it's almost certainly a problem with the follower, the spring, or dirt inside the mag tube.