try going back into walking sands or leveling up. Every story quest leads right to the next story quest. If its not from the person you turnt it into, its a npc in very close proximity. Keep in mind, sometimes the story quest will jump a level, so while you may have been high enough to receive that level 25 quest, the followup quest could be level 26 and you wouldn't be able to receive it.

For whatever reason right now, some quest givers are sometimes not showing the red quest marker above them when they offer a quest for you once you've leveled up. I've experienced this personally when I dinged near Quarrymill and tons of level 26 quests just appeared without any indication beforehand that they would become available then.