I'm not a 1911 ghuru,but if the extractor is not messed up,think about the slide moving forward and stripping the round.If the bolt face is in contact with the round,the round cannot get aheadof the extractor.
Hmmm,
But now imagine that round is only held in position by the mag spring against the feed lips.Think of the rounds inertia,and the gun literally recoiling out from under it.Then the round has moved up and forward while the slide is still back.
The extractor will be what drives the round forward
Options:
New mag spring,Wolfe +10%
Do you have a dimple on the follower? That is what it is for,last round retention.No dimple? Look at other mags.
If you are loading hot,or if your recoil spring is fatigued or softball,your slide may be smacking back harshly,pulling the gun out from under the round.
A very soft ,relaxed hold can contribute.Hold at least a very firm handshake.
One more idea,the slide stop.The follower lifts the slide stop as the last round feeds.That means the slide stop and the follower engage on the last round.Given,everything on a 1911 requires fitting,it might be a left thumb contacting an extended slide stop is blocking the follower rise.