do supermarkets really have issues or is it more that they only have 1-2 brands instead of about 5 different versions of the same thing?
One of the first signs that the supply and delivery chain is having trouble is something like supermarket chains cutting down on choice...
Apparently the plan is they'll reduce the lesser selling brands first to keep the maximum number of customers happy (so they'll say try to keep the most popular booze in stock but drop lesser selling brands even if those dropped brands are a higher profit margin), then they'll do things like reducing the number of multipacks because someone might buy 4x individual packs of crisps rather than a pack of 6, but they won't generally buy a pack of 6 if they just want one (I think there is also an element that for some items the delivery boxes for singles take up less space for the same total number than the multipacks).
Effectively it's triaging the stock to try and maintain the most common items when there isn't the capacity in either drivers or at the ports.
One of the biggest dairies in the country was reporting a week or two back that they'd not been able to deliver to something like a third of the supermarkets they supply because of a shortage of drivers, at the same time our milkman suddenly stopped delivering (which is extremely unusual in the 10 years we've had him*), and when we ordered some to be delivered as part of a Morrisons home delivery they subbed it out for cravendale (which I noted had a much shorter date than later deliveries).
I've noticed an increase in things in our grocery deliveries getting subbed, and certain things being out of stock for much longer than normal for a few months, things that were fine during the worst parts of the pandemic last year.
At this point I can quite believe that the reason McDonalds doesn't have any stock for milkshakes may well be that a dairy supplier is prioritising deliveries to supermarkets or for other more essential milk based products.
*At this point I'm not sure if he's gone bust, ill/had an accident or supply issues as I had noticed his deliveries over the previous few weeks had been from two or more dairies (rather than his normal one) and often shorter dates as if he was struggling to get stock. Unfortunately he's never been good at communicating.