A spell from that wuss Skeletor is different from a spell from King Hsss (poisoned the Sorceress) or Hordak (separated the planet).
Granamyr probably couldn't change Man-At-Arms back if the Sorceress couldn't cure herself with all of the magic tomes of Grayskull at her disposal. King Hsss' magic is as ancient as the Elder's. He-Man had to time travel in order to discover the secret of how to beat Hsss, knowledge that Granamyr probably doesn't have. Besides, the Dragons of Darksmoke were at war with King Grayskull and the Snake Men, proving that King Hsss has staying power against Granamyr.