Why you disappointed with yellow? That means nothing. Looks like clean yellow fluff.
As for what you did wrong, probably nothing, just because you have bark doesn't guarantee a 2% yield of alks. There are many variables in bark yield, sometimes bark won't yield anything if it's bad (misleading supplier's),time of year harvested, bad extraction techniques, etc.
If you go into an extraction with big expectations of huge returns, then you're probably going to be disappointed. With acrb, you're lucky if you hit 1% because again it varies so much.
Be thankful and celebrate that you have some to work with, follow the tek to the letter and you'll get there with better yield, possibly, but never count on it.
Keep up the good work
Safe and happy journeys friend
Peace