Every year the Grateful Dead give an MP3 download a day in November. I downloaded all of these and copied them into iTunes on my Mac and they all play perfectly. I made a playlist and copied it to my iPhone 6 but two of the tracks do not play, they are greyed out on the phone and I get an error message "Item Not Available - This item can't be played". I have tried repeatedly to reload the tracks but without success. Any ideas please?