Shopify Cart Expiring. Need to stop it from doing this.

Description

I do not know value of work that needs to be done so I have set it to £20.00 to guess
Customers keep reporting that they add items to their cart, login 5 days later and their cart is empty...
This is resulting in loads of business loss and customers getting frustrated.
Customers need to be able to login from PC, Mobile, Laptop, iPad, etc and see their previous cart and be able to add to it without it expiring or disappearing.
We do not sell the type of items where customers come on the site and make orders the same day, most customers will build their carts over time and then make one big purchase - they cannot do this because the cart is expiring too quickly!
Customers are accepting the cookies no problem, but its still causing an issue.