Ok so there is this mechanic in place: when u fillet fish and get more than 10 fillets, you will get no butchering skill for the action. This kinda makes sense because some fish give 50 fillets and the skillgain is measured per item produced, so it would be huge.
However it feels very unintuitive when you have carp that gives 11 fillets and you get no skill for it, so you need to lower the quality somehow so that it gives 10 fillets instead.
My proposition is that every fish should give skillgain for the first 10 fillets from the action. No matter if it gives 12 or 50, you will get skill for first 10.
Since you are still butchering, why aren't you getting any better at it o.O