We're a medium-magic setting where magic is fallible. What this means for you:
- With a grave enough wound or a long enough time dead, a Resurrection cannot bring the subject back to life
- Against a grave enough disease, Cure Disease isn't enough to cure it - only to treat symptoms and alleviate pain, at best
- Remove Curse does not work against every curse, nor should mages and clerics expect it to
- High-level spells tend to come with some form of downside: physical weakness after casting them, etc
- Legendary enchantments tend to come with some form of downside, see our loot guide
The failure of a spell in a DM event doesn't necessarily mean you're up against even stronger magic. Keep in mind that it might have been an especially grave wound, a particularly chancy challenge, or a casualty beyond saving. Failure to resurrect a player (due to the player's OOC choice) doesn't have to be played as the soul's refusal to return; it could just be the severity and duration of fatal wounds.