Think you must just be unlucky - I've ordered quite a bit this year - always delivered next day, even when ordered late on the evening before - might depend on where you live.
These are the three.
Ordered a Kindle for next day delivery to Amazon locker at work. Expected time was 4pm. This changed to 6pm, then to 8pm. I waited, tracked, contacted them and was told the courier was nearby. At 8pm they delivered to the locker, but my item wasn't there. I waited all day for this. Was fuming. It came the next day.
Ordered a Shark vacuum cleaner to be delivered to home. We waited in all day then it got cancelled by Amazon. They said it was out of stock. It wasn't out of stock when we ordered it. We were going away the day after so couldn't wait for delivery.
Ordered a cut throat razor on Monday this week for Tuesday delivery. Tracking said it was due in the afternoon so we waited in all afternoon. The next update said they attempted delivery at 6:09pm. I was sitting in my living room watching City at the time, and they definitely didn't ring the bell. If they knocked, which I doubt, it was a soft knock which we didn't hear. I was facing the drive and nobody came that I saw. We live in a cul-de-sac (in a village) and easily see all delivery vehicles. Nothing came. They said they have the coordinates of the delivery attempt, which matches my address. I have my doubts.
I had a four hour web chat with them to resolve this. They promised delivery by midday on Wednesday. They phoned me at 11am to say they would deliver between 5pm and 7pm. I told them to cancel as I would be out visiting my auntie in hospital.
I have written a letter of complaint. They have twice replied to say the order has been cancelled and refunded. They haven't answered any of my concerns or provided me with transcripts of my web chats, as requested. They haven't got back to me with details of the attempted delivery, as requested. They haven't applied £10 compensation credit to my account, as promised. They haven't refunded one month's Prime subscription, as promised.
On the Kindle failure I had a long complaint with them. They answer questions they think you want to know, but never answer the questions you are actually asking. I find them great most of the time, but they can mess up spectacularly.