What Magazines are you using?
Try a different Magazine or two. Most malfunctions of this type are magazine related.
I've also seen this kind of failure due to an accuwedge between the upper and lower trying to remove the "play" between the upper and lower.
Essentially you're putting more gap between the carrier and the magazine causing issues stripping the next round from the mag and getting hung up as it tries to feed.
You also want to check your magazine catch for abnormal wear.