Most would be surprized how many 1st class mailings are returned by the USPS for  not deliverable to the address on their envelope, even thought the same mailings have went to whatever the address is for years.

Who knows why.  New or temporary letter carrier, most likely.

I have gotten magazines returned for that reason even tho the member has lived at the address with no changes for 25 years.

The larger the post office the more inept the employees work there.
Small towns have no problems, most the time.

Go to the USPS web site and input the address in the zip code look up and it will give you a clue if the address is ok.