Asda complaint: Online food shopping

I have ordered all my food online since the Pandemic and have continued to do this. Unfortunately, you are at a distinct disadvantage when ordering food online and this is very unfair.
I have placed orders with ASDA, TESCO, and Morrisons, and experienced the same problems. When you order online you book a slot in advance (which you pay for) and then order your products. I understand that there will be a more restricted range of products available for logistical reasons. What does annoy me is that you can book a slot two or three weeks in advance, fill your basket etc, only to be told at the last minute that many of your chosen items are unavailable out of stock.

This is a niggle on everyday shopping, but when we come to Christmas time, it gets considerably worse. The major supermarkets are eager to release Christmas shopping slots in mid -November, and i have fallen into this trap to my detriment. When your Christmas order is delivered you find several important seasonal items are no longer available. This year I have been placing orders online, only to find items are leaving my basket almost as fast as I’m putting them in.!
I feel like a second – class shopper, where I can have what’s left on the shelves after those who shop directly in the supermarket have finished. Ordering well in advance obviously means nothing, and I have been let down and disappointed on several occasions.
I thought the system was more sophisticated than it obviously is, and perhaps I’m expecting too much from it. At the end of the day, the supermarkets aren’t bothered, and it’s just your hard luck!