PARAGRAPH 1: reason for writing
I am writing to complain about/regarding/because of/on the subject of.....
I am writing to you in connection with......
PARAGRAPH 2: complaints and justification
Although I have only used the washing machine once, it no longer works. I have called your shop twice but you have not taken any action.
FINAL PARAGRAPH: suggested actions to be taken. Closing remarks
I assume you will replace the washing machine. I hope the matter will be resolved promptly.