Hi,
I've just swapped to a new PLA (Sunlu wood filled) and I'm getting strange lines in my top layer.
It is like some areas are just not being extruded at all.
Any ideas how I go about finding the cause of this?
Looks like classic wood filled filament clog. You see standard plastic is like water flows well this has sawdust mixed in so it's more like peanut butter. Check for overheating it is probably turning into charcoal and sticking or the nozzle is 0.4mm so it's clumping and if you are in humid environment it could just be absorbing moisture that could be it too.
I would try with 0.6mm check if it feels moists try with ironing off and see it is running too hot to make it like this.