Newb with a problem

I'm having an issue with my M&P Sport II; I've already scoured this sub for answers but I'm shit out of luck so far. So here goes:

Last year I got my first rifle, this Sport II. Without cleaning it or doing anything to it or adding/swapping any parts, I took it to the range a couple of times within my first month with it, shot less than 200 rounds through it, and everything worked perfectly. Then last week, after months of not firing it, took it to the range again and started having what looks like undergassing: first round will fire just fine, but the next round fails to load; it looks like the bcg doesn't go back far enough to push the next round out of the magazine. Same ammo as the first time I used it, too.

That same day at the range I doused it in oil, and the problem continued. So after reading here I figured maybe I had gunked it up by not cleaning it before using it the first time, so I did a proper cleaning, after watching dozens of youtube videos; I pulled the bcg apart and cleaned everything as best as I could. Then tried the rifle again and the problem persists.

Apart from external accesories, the rifle is stock; nothing's changed in it. So what could be the problem? What else could I try to at least diagnose it better? I appreciate any help and advise.