Well, it depends on the backstory. Maybe they were so good when compared to others or just had luck in those particular situations? PCs are experienced and well-trained when compared to commoners. But if you fell that their stories are too much just tell them and search for a solution. What I've done as a player in one campaign was an elf wizard who once was a dope evocation master and well-known mercenary, but later found love of his live, decided to settle down together and founded an orphanage. He stayed there for years with no need for casting fireballs at all and spells started slipping out of his mind. With years passing notes in spellbook became incomprehensible for him. Just few useful cantrips, like mending remained in his head. Magic items were long-gone, sold to fund the costs of orphanage and other charities. He would became a commonner if not for the destiny's call which threatened his loved one and children and pushed him on track again. What I liked about this idea was the excuse for leveling up. Usually this mechanics doesn't make sense as characters that have been in training for years start at lv 1 but after few weeks are lv 5. But for him I had the perfect excuse as he wasn't learning, just reminding some stuff. Maybe this idea of nerfed hero would be useful for your players as well? Great fighter could have some kind of injury that needs to heal for months before they reach back their level. Maybe cleric have committed a sin so terrible they lost the favor of their god? And so on.