Did you try the "bad" rounds a second time? If so, did they fire then?
Here's my wild theory.
It's a new gun, and you haven't taken it apart to clean it. There's some factory grease/gunk on the rails, so when you single-load and let the charging handle go forward, it's not going all the way into battery (all the way forward). When that happens, the firing pin doesn't get struck hard enough. However, when you're multi-loading, you only have this issue on the first round, as all of the other rounds the charging handle gets "slammed" back and forth from the energy of the fired cartridge.
Just in case my theory has any merit, I have two suggestions:
1) Take it apart and clean it.
2) When loading a round from the mag, pull the charging handle back all of the way and then "let it fly." Don't pull it just part of the way back, and don't let your hand ride it as it goes forward. LET IT SLAM.