My native language has a saying. Its literal translation would be:
"You have finally caught a mouse since you became a cat."
It is usually said when someone usually a man has finally succeeded in something important, or did a very good job or did exactly what is expected of him, although he never seemed so, or he seemed to have failed all the time and never achieved what is expected of them.
For instance, if a young man, working in his father's shop, has never managed to sell anything to any customer although he has been in the shop for months, and his father's hopes have been diminishing, but someday he unexpectedly sells a very expensive product and brings good profit for the first time. Obviously, the father, astonished, says this happily to encourage him: "Son, you finally caught a mouse ever since you became a cat."
So, I wonder what would an equiavelent saying for such situation in the English Language?.