With Royal Mail introducing USO reform and new delivery methods that are designed to reduce costs, some mail can now legitimately be deferred while parcels and mail due for delivery are prioritised. However, that doesn't mean that you should accept going days or even weeks without mail that should have been delivered to your door.
If you are experiencing persistent delivery problems please make an official formal complaint to Royal Mail:
I wish to make a formal complaint about repeated failures to deliver mail to my address. We have experienced periods of several days without deliveries. Please record this as a formal complaint and provide me with a complaint reference number.
Royal Mail Customer Services 03457 740 740
Or write to: Royal Mail Customer Service Centre, FREEPOST ROYAL MAIL CUSTOMER SERVICES
Keep a record of the dates on which you believe a delivery was missed and any important mail that arrives late.
If Royal Mail's complaints procedure doesn't resolve the issue, then Ofcom confirms that eligible unresolved complaints can be referred to the independent Postal Redress Service (POSTRS).
https://www.cedr-assist.com/consumer/postrs/overview
Ofcom doesn't normally resolve individual Royal Mail complaints itself as it regulates the overall postal service and monitors Royal Mail's compliance with its Universal Service and Quality of Service obligations. So if your deliveries are persistently failing please complain and get it recorded.