Well unfortunately things in this game, I believe characters are included, will decompose in time. If you drop something on the ground and leave it that item will eventually disappear. Does the dead characters portrait still appear on the right side of your screen? It is possible to take so much damage that the image will disappear and that character can not be raised or resurrected. Secondly, what spell are you trying to use? Raise Dead or Ressurrection? Some races can not be raised and must be ressurrected instead.
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The line between good and evil is razor sharp. Be careful of misteps,
lest you find yourself spitted upon your own blade.