Maybe they can’t employ more people because they are not available for employment. Lorry drivers etc are in very short supply right now. Many of our local supermarkets are having trouble to even fill the shelves due to staff shortages.
We have our shopping delivered by Tesco every week - just had a delivery now - and are always delighted with what we get, including substitutions which we are generally very happy with. Of course we pay to have it delivered , but it would cost more in petrol and our time if we went to the shop and loaded our own trolley. We tried several delivery companies for our shopping during the first lockdown and settled for Tesco as the most reliable and efficient. I think that generally it’s easier to get slots now with all the supermarkets. Maybe it’s time to try a different company if you’re not happy with the service you’re getting.