Last round didn't cycle - Alexander Arms .50 Beowulf 4 Rounds Magazine

I bought the 4 round mag as well as the 7 round mag. Took them out shooting and the last round of the 4 round mag would never cycle correctly - round always dead headed into the breach face of my BCA upper. Never once did it cycle the last round correctly. On the other hand, the 7 round mag cycled flawlessly on all and the last round.
I originally bought the 4 round as it was fairly flush and didn't want a longer mag when bench resting. However, the 7 round mag is only maybe two inches longer so no problems bench resting. I am returning the 4 round mag and will get a couple more of the 7s.