Double needle hem on jersey keeps waving back after I press it flat Machine Trouble

You're stretching it, and reducing pressure alone rarely fixes cotton-lycra. Three things together sort it out for me.

Drop the presser foot pressure further than feels right. Most machines go much lower than people try and the feed dogs still move the fabric.

Stop pulling behind the needle. Guide it, let the feed dogs work, and if you must apply tension apply it equally in front and behind so the fabric passes through relaxed rather than stretched.

Use a stabiliser strip in the hem allowance. A 2cm strip of lightweight knit fusible along the fold gives the double needle something stable to stitch into and eliminates the wave. It slightly reduces stretch at the hem, which for a skirt is fine.

The wave returning after pressing means it was stretched while stitched, so the stitching is holding it longer than the fabric wants to be. It'll never press out permanently.

Check bobbin tension too. A double needle hem forms a zigzag underneath, and if the bobbin is too tight it pulls the two lines together and tunnels the fabric, which reads as waving from the top.

If your machine has adjustable presser foot pressure but you can't find it, check the manual for a dial on top of the shank or under the top cover. Mine was a screw I'd assumed was structural for two years.

Let the cut piece hang for 24 hours before hemming. Cotton-lycra grows under its own weight, especially on anything near a bias curve, and if you hem it straight off the cutting table the length is already wrong.