I have had it happen when there is something wrong with the address. The computer checks the address to be sure it is accurate and complete. The post office has some odd rules about addresses. For example they will not allow St Louis, they insist on Saint Louis. Yet they will not allow Saint Petersburg, they insist on St Petersburg. Note the lack of consistency.
It might be that the address is an apartment building and the apartment number is missing. Or the ZIP code doesn't match the city.
There may be other reasons but I have found that is usually the problem.
