Remus was a good student and excelled in a lot of subjects. But he never really was up to the mark in Potions.

He confessed that seeing all those weird ingredients made him dizzy at times. And that he never had the potential of brewing a potion accurately. In class Remus had exploded numerous cauldrons and struggled to scrape an Acceptable in Potions in his OWLs exams.

That is exactly why he wasn't able to brew the Wolfsbane himself when he was teaching at Hogwarts. Wolfsbane is a highly advanced and complex potion and a slight mistake could render it useless. 

And according to Remus, he was the epitome of mistakes when it came to potions.