Several of mine kinda jam up when you hit 18rds. I think the follower can catch on the transition between the tube and the extension. Try pushing the round down a few times, or tapping it on a table while you load. Once you get 19 in the 20th goes in easy. Once you get it to load a few times, it gets easier.
Like someone else said, if the mag jams up halfway through while shooting, round all the edges on the follower. I had many headaches before I figured this out.