First be sure the parts in the mag well are oriented properly the spring the button and the catch.Remove the stock and familiarize yourself with how everything operates Then put it back in the stock and
There is a little tab on the side of the mag that catches in the mag well. It may need a little tweaking.carefully bending it out a very little at a time may help. Don"t bend it too far or it will make the mag sit too low in the mag well and the cartridges wont be picked up when cycling the bolt. also careful not to bend it back and forth repeatedly as it may break. Just a little tweak should be all that's necessary.
I would use something flat to spread the load between the mag so you only tweak the part you want and a small screwdriver and pry just a little at a time on the tab and check it.
Good luck

Spring he's referring to should be a weird shaped one in the mag well.