Unhappy employees will never be truly happy. There is nothing you can do to make them happy. They will eventually quit or you will fire them for a legitimate reason. To cut your company's turnover rate, you must first accept this simple truth. I learned it the hard way. You don't have to.
Twenty years ago, I was asked by a retail company to do something about their high turnover rate, which was about 50%, half of the company's entire workforce was leaving every year. The executives were tearing their hair out. Salespeople and administrators alike were quitting in droves, year after year. The human resources department did little else than recruit and interview people they knew would leave the company in a matter of months.