Of the 10 MKII mags I have I had two that exhibited the same characteristics you speak of...I found that the slot up the side of the mags had a pretty severe burr on the inside edges that would ride and grind on the inner shoulder of the follower button ,,,not the button itself, but the shoulder area that goes through the follower. I used a round needle file to remove the burr along both sides of the slot blew it out good to get rid of filings and they have worked fine since. Look at the follower button and mag slot for tell tale evidence of scrape. it seemed with these two mags anyway that with 10 rounds against the spring the follower was cocked just enough to allow the button shoulder to scrape
good luck Aimhigh