Five useful ways to complain in English politely.
1. I am sorry to bother you.
2. Can you help me with this?
3. I'm afraid there may be a misunderstanding.
4. I understand it's not your fault.
5. Excuse me, I understand that.

Stay cool when you are making a complaint.  Speak clearly but politely.  Organise your points and speak coherently even if you are very angry over a situation