Why does my seam look fine until I press it?

Pressing does not create the fault, it reveals it. A seam under the presser foot is held flat by the machine; off the machine and under an iron, the fabric relaxes and shows what the stitching actually did to it.

The usual cause is tension too tight overall, even when the stitch is balanced. A balanced but tight stitch draws the fabric in slightly along the seam line. Flat on the table it is invisible; pressed, it reads as a rippled seam that will not lie down.

The second cause is stitch length. Short stitches on fine fabric perforate a line that behaves like a gathering row. Lengthening slightly, to around 2.5 mm, often fixes what looked like a tension problem.

The third is fabric handling. If the fabric was pushed or held back as it fed, the seam is under stress before it is pressed. Let the feed dogs do the work and only guide.

The fourth is the iron. Pressing along a seam, rather than lifting and lowering the iron, drags the fabric and creates ripples of its own. Press, do not iron.

And on synthetics, heat. Polyester and its blends distort at temperatures cotton takes happily. Test on a scrap.

The check that separates them. Sew a test seam, press it, then compare it against one sewn half a number looser and 0.5 mm longer. Whichever presses flat tells you which variable was wrong.