I used it against Calem/Serena's Clefable in Y, and it's been over three turns and nothing has happened. I tried using it again, but then it failed. Why isn't Leech Seed working? Is it because of Clefable's ability, whatever that may be?
Calem/Serena's Clefable ability is Magic Guard meaning it only takes damage from attacks. Moves like Leech Seed, Toxic, Will-o-Wisp, etc. will not do any damage to it whatsoever.