Unless absolutely necessary.
The supermarkets are busy. They know there are hardly any delivery slots and that some items aren't in stock. Contacting them just to complain about these issues is causing the staff extra work, when they're already busy dealing with queries about orders.
Don't tweet them or contact them on Facebook about stuff that's already been posted, e.g. Iceland having a dedicated hour for elderly shoppers. Again, extra work when they've already been contacted about it.
Please just think about the staff dealing with all of this, cut them some slack and only contact them if absolutely necessary. It'll make their lives a lot easier.