Serger Stitches: Flat-locking
Try stiffening the organza with some liquified water-soluble stabilizer scraps. I think because the organza is limp/wimpy is part of the reason you are having issues. Try increasing your lower looper tension. My 643D needs the lower looper to be between 7-9 to give me a good stitch. Needle is lowered - so 1 or 2 should work. The upper looper a low number as well at 2-3.
I always keep my scraps of WSS so I can put some in a small container and add drips of water at a time to make an "Elmer's School Glue" consistency. Paint it on and let it dry. Wash it out when your project is done. And- if you add a couple drops of rubbing alcohol as the liquid when making your "glue" you can keep the mixture in the refrigerator for a week or so. Just throw it out when it turns green as you do not want to keep your sewing version of penicillin when it's green. And- you may need to label the container so no one at your house thinks it's the beginnings of a sauce or gravy! (big giggles but could be true with the holidays coming!)
Tracy in Nashville, TN
Martha Pullen LIcensed Educator