Magic-based healing only heals the last damage taken. When your team members take damage, mouseover them and you'll see "wounded: 8" or however many damage they took. That's the wound that a healing spell can heal. If it's not healed and he then takes another 1 damage after the 8, his wound will be "wounded: 1" and you'll only heal that 1 point of damage. That 8 damage now can't be healed through magic. You'll need to use a medkit to heal it.

All of your party members' wounds are automatically healed after combat, so if you're careful not to take damage more than once in combat, they'll effectively not have taken any damage in combat.