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.