User blog comment:Morganaforever/Episode Review: The Diamond of the Day Part 1/@comment-5829707-20121223054011/@comment-86.144.141.102-20121223103433

Perhaps if Merlin is magic, then the slug sucked out a good deal of magical power from him before it was removed but couldn't actually steal all of his power, just enough to prevent him from using magic at that time. And maybe once he failed the first few times he assumed that it was gone completely and didn't believe that he could do anything?