Why is it that some of the music I buy on my iPhone turns gray and doesn't work after time?

My family all uses this Mac downstairs and we buy music on my dad's apple account. I bought some instalok music and other random music, downloaded onto my iPhone with other music. It all works and stuff, but then a week later or something the text changes from black to grey color and it doesn't do anything when I tap it. How can I fix this?

Try resyncing your music on the iphone