the musical “Parade” and Tom Stoppard’s play “Leopoldstadt.” The problem is that while “Harmony” is about a sextet of singers whose voices blend like milk and coffee, its elements do not similarly fuse into a cohesive and satisfying musical.The show, directed and choreographed by Warren Carlyle, has been tinkered with by Manilow and lyricist/book-writer Bruce Sussman for nearly 30 years, but on its largest stage yet it still doesn’t quite work.Structural flaws that were mostly forgivable when the production played the more intimate Museum of Jewish Heritage downtown last year are detrimentally exacerbated by Broadway’s imposing size. The musical, therefore, is lopsided.