Why did I lose 4 golden achievement balls from yesterday to today?I practice daily!
"Achievement balls"? Do you mean the skills on the tree? You probably need to practice more. Either way, those skills are designed to go grey so that you come back and practice the skills again. It would be pretty boring if they remained gold forever.
Use the strengthen skills button for that.
More info explaining what Nuno already mentioned:
If you keep strengthening the skills (click on each one and press the "strengthen" button), and assuming you mostly get the answers right without too much checking translations, they will stay gold for longer and longer until they stay that way for ever. But you have to practice a lot.
Do you think they really know when you hover over the words for a clue? That's great if they do. It forces you to study harder as you do the lessons.
I've heard that they do record the number of hovers and use it to influence how soon a given word gets shown to you again.
But I don't know that for a fact.
I've heard about that too... but never read a confirmation.