Supermarkets are struggling to identify vulnerable customers in most need of home deliveries during the coronavirus outbreak.
Most major retailers which run online deliveries say they want to give priority to those in greatest need.
Booking slots are released for two or three weeks time, but they are being quickly snapped up due to high demand.
Some retailers, including Waitrose, have closed their websites to new bookings while they try to prioritise.
Sainsbury's has been in touch with loyalty card holders whose birthdays are on file, showing those who are aged over 70.
It is also trying to use the list of those English people in the "most vulnerable" category. In Scotland and Wales, where there is no central list, they are inviting people to call by phone.
This week, Tesco joined others with a recruitment drive for thousands more pickers from shelves, as well as more vans and drivers.
The company, Britain's largest retailer, says it is increasing both home delivery and "click and collect" to about 780,000 slots this week, up from 600,000 two weeks ago.
It plans to increase this by a further 100,000 in the coming weeks.
Morrisons is recruiting 2,500 pickers, expanding the number of stores covered by its delivery service, and setting up a phone service for those who do not want to order online.
The Scottish Retail Consortium warns the capacity for home delivery is a small share of the total demand for food, and supermarkets "can only do so much".
Its director, David Lonsdale, said: "It's up to friends, family, communities and the authorities to rally round and make sure vulnerable people get the products they need."
Local independent shops have responded strongly, starting or expanding home deliveries through the crisis, often in communities where they know their older and most vulnerable customers.
Several vulnerable people have been in touch with BBC Scotland, to say they have been frustrated in their attempts to get deliveries.
Scott Cormack has been through heart surgery and a prolonged infection in hospital, before returning to his home in Point, on the Isle of Lewis.
He has been told to isolate, and tried to get food delivery from Tesco, the only major supermarket brand on the island.
After 10 days, he still had not secured a delivery slot.
He said: "They are not prioritising anyone in my condition, with underlying issues.
"This is about to kick off up here, and if they don't change the way they're operating at the moment, it will cause a lot of damage."
In Glasgow, Steve Briggs, who has a serious heart condition, said he could not get a delivery from any supermarket within 50 miles.
Until a neighbour pointed out that the Iceland chain had a slot available, the first shop they could secure, without going into a store, was a "click and collect" booking in Falkirk three weeks later.
He said: "It's not been safe to leave the house, which means we've been reliant on what we had in store at the point it became apparent that this was hitting the UK very hard."
Asda is expanding capacity, but has not found a way to prioritise vulnerable customers and ensure they can limit abuse of such a system.
A spokesman said the company was doing "all it can" and encouraged customers able to visit shops to think how they could help others in their community.
Among other major retailers, the Co-op recently began a trial to deliver with Deliveroo, which is better known for bicycle delivery of hot meals.
However, more local-based stores have been stepping into the breach.
The Viral Kindness project - set up to help vulnerable people across Scotland - has expanded after joining forces with the Scottish Grocers' Federation.
The scheme launched a week ago with 300 Day-Today stores and 200 from the USave group but now has almost 600 participants.
Shopkeepers have been making contact with thousands of local people in a bid to tackle loneliness and to ensure they get shopping deliveries.
The organisation's free 24-hour helpline, website and digital channels have been making connections between people and organisations.
About 250,000 Viral Kindness Scotland postcards are being distributed to stores by the Sarwar Foundation to identify vulnerable people in their local communities, which can be returned to the charity by freepost.
An army of volunteers are delivering the shopping packages to vulnerable residents and offering a friendly chat.