Zig zag stitch not stitching properly

[Thesockthief]

I have a Janome machine and the zig zag stitch isn’t coming out as a zig zag. See attached photo. I’ve re threaded, I think correctly, several times and it starts stitching correctly but quickly ends up like the photo. The back does look like a proper zig zag.
i also changed the foot and the needle seems to be sharp. I’m relatively new to sewing and not sure what else to check. Straight stitch is fine. Can anyone help?

Tension / stitch length / correct needle for fabric thickness - worth checking

Tension

Tension - what are you sewing? Looks like carpet! Might need heavier duty needle.

Ha ha. Definitely not carpet. It’s a kind of velour fabric. Reasonably thick though so yes, possibly the needle (although it’s ok on lots of the other stitches on the same fabric). Will definitely check the tension too. Thank you!

That looks to be a tension problem. The bottom thread is loose compared with the top thread. You may need to increase the tension for the under thread bobbin.

Thanks. I’ve no idea how to do that but have experimented and switched tension to 2 and it seems to be working. Thanks everyone.