I thought recently that my Janome DXL603 was stitching smaller than it should sometimes, and skipping the odd stitch, usually when sewing through a few layers eg zip and double layer of interfaced cotton. Then it would be Ok again so I ignored it. But I did sense that something wasn't quite right.
Today I was sewing through several thick (but not heavy) layers of upholstery cotton and Bosal foam interfacing. It managed 3 seams with just the odd skipped stitch but when I sewed the fourth it skipped massively. I tried
Changing the needle
Rethreading
Increasing the tension
Changing to a walking foot
Swapping the Moon thread for Gutermann
Cleaning under the needle plate (nothing much there, I did that recently)
No change at all.
As a last resort I tried it on my old Singer and the seam was fine! I thought the stitches looked better too - not so tight, more defined, despite the difficulty squeezing the fabric under the foot.
Any idea what my Janome's problem could be? I'd rather not take it for a service if it's something I could easily adjust myself. Could it be the presser foot pressure screw? That's always been a mystery to me and I seem to have tried everything else.