Seeing that the lower is a jig build I'd bet money that the mag catch is not in the right location. I think and am almost certain it's too low but it could be high depending on how exactly it's jamming.
Load 3 more in the magazine, sling shot the charging handle, post a picture of the jammed gun, through the ejection port.
Load 3 more in the magazine, sling shot the charging handle, post a picture of the jammed gun, through the ejection port.