I think a lot of it is down to stock control in the warehouse, you can order something that is shown as ‘in stock’ but when the picker goes to get it, it isn’t where it should be.
At this point your order becomes stuck in a processing limbo until:
1. They find it and get it sent.
2. They don’t find it and the item goes to out of stock.
3. You properly chase them to get it sorted (at which point either 1 or 2 happens)
It’s a shame, they used to be really good with their stock control, but something seems to have gone awry recently.