Are you gluing the fabric to the foam before sewing?

@Vincent De Gronckel
Other then experience, it also depends on your machine you're using and how its setup(pressure on walking foot and how you're holding material/feeding). With that said I see no top stitching and assume its going back to OEM Design. I'd only use 3/8"-1/2" for middle inserts. 1/4" for side bolsters. Since its fabric I'd spray just enough glue on sew foam only to adhere fabric (like a quick line) just enough to
to hold for perimeter sew not permanent (1/16") fabric to sew foam. After that make sure you have sharp shears to cut excess sew foam off at 45 degree angle(fabric on top & sew foam underneath). Angle shears under fabric for 45 degrees. That should help to see what you're doing with out excess sew foam.
Let the machine do its thing, dont hold too much tension and dont feed too much. practice makes semi perfect. Good Luck