Conversation so far:
What was Frank Sinatra's first debut?
Frank Sinatra made his film debut performing in an uncredited sequence in Las Vegas Nights, singing I'll Never Smile Again with Tommy Dorsey's Pied Pipers.
What was Frank Sinatra first musical film?
Frank Sinatra made his film debut performing in an uncredited sequence in Las Vegas Nights, singing I'll Never Smile Again with Tommy Dorsey's Pied Pipers.
What was another of Frank Sinatra's musical films besides Las Vegas Nights in his debut career?
Frank Sinatra had a cameo role along with Duke Ellington and Count Basie in Charles Barton's Reveille with Beverly, making a brief appearance singing Night and Day.

Question: Why did Sinatra's career slump?
Self-contained rewrite: Why did Frank Sinatra's career slump?