I have a M39R as well, and I fixed a similar issue. Following your link, the part that Amorton94 circled is called the locking block, make sure that hasn't broken the part of the barrel it pivots on, or has otherwise fallen out of alignment. Also on Amorton's last picture, I noticed his barrel is very badly worn on the top where it contacts the slide, this has happened to me before and it causes lots of non-obvious problems with the slide not going back into battery.
Both of these have happened to me and I solved them both by just buying a new stock outer barrel.
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