A.Liadov (1855-1914), might just fit into this category. His few compositions are gems, but what might have been ?, He probably qualifies because of his reluctance to apply himself to his work, with the result his output was minimal. Colleagues encouraged him & tried to prod him to action. He was dismissed from the St.Petersburg Conservatory because he refused to study.Most would agree that he had the ability & potential to become a great composer. He was an alcoholic, but his lack of self confidence may have been caused by untreated depression, a condition that was not dealt with, or acknowledged in those days. Diaghilev aprroached him for a score for "The Firebird", but because he took so long to answer, and seemed to show no interest, the commission was offered to Stravinsky. I would imagine the Liadov score would have been very different to Stavinsky. Our loss ! We can only wonder.
Marcus