I think maybe its one of those things where it's easier to do it if you are certain you can do it. Like flying, right? When you show the broom your in control it does what you want ever so much easier. When you just beleive you know how to make the potion, it just works better.

Was your mum a really good Potioneer? My brother's brilliant at potions, but he hates having to make them for his Healing programme.