Properly Addressed Mail being Returned as "No Such Address"

For several months now, my mail has been returned to sender marked as "no such address." I have lived at the same address for six years, with no change to location. I've called USPS customer service, talked to the local station manager twice, and even gone in to the office with an example of a returned envelope. However, my mail is still being returned to sender. Anyone else have this problem and have any suggestions of how to work with USPS to get mail delivered? In the interim, I've taken to providing everyone with the 9 digit zip (5+4) and even then, not all mail is getting through to me. Open to any suggestions.