It had been many months since Remus had transformed with the help of the Wolfsbane potion. While teaching at Hogwarts, Severus completed the brew and kept up with it until the day he murdered Dumbledore. It took a skilled potions master to complete, and the slightest mishap was potentially lethal. Remus had not necessarily been a poor potions student, but he certainly never excelled like Snape or Lily. Sirius barely skated by with a qualifying NEWT in the class to get into the Auror Program, so he was back to full, unaided transformations.