This letter is a follow-up to the first bad service complaint letter, to which you received no reply. Be firmer but do not threaten...yet.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
<time you wrote the letter> I wrote you a letter expressing my dissatisfaction over the service I received <description> <time>. I requested some kind of explanation of how you would remedy this service situation. Since that time I have received no reply of any kind, so I am contacting you again.
I wish to remain a customer of yours, but I am currently very unhappy with the way I was treated by <employee(s)>. Please respond immediately so we can resolve this matter to both <company name>'s satisfaction and mine.
I look forward to hearing from you.
encl: <List of enclosed items goes here>