Yes there are reasons where the post office or USPS can refuse to deliver your mail to you.
For example if you have a mailbox at your door and you have dogs that try to attack the USPS workers when they deliver your mail then they can refuse to deliver the mail to you.
Or if you have a mailbox at the street and it's full of mail or it's not at the right height or not in regulation with the USPS then they could also refuse to deliver your mail.
If someone is blocking your mailbox then that's another way that USPS can refuse to deliver your mail to you.
Most times the mail will be delivered to you without any issue but just make sure you get your mail often enough so that your mailbox is not kept full and make sure you keep any pets away from the delivery area where the USPS workers are gonna need to walk to your door to deliver your mail.