The fatal blow to the Green Shield Stamp was the loss of Tesco as a participant, even though other small chains such as International, Gateway and Budgens ran with the idea for a couple of years.
Interestingly, Lord Sainsbury refused to have GS stamps in his stores, branding them a gimmick and masking the true cost of products. (I wonder what he would have made of nectar points...)
The problem with trading stamps was that, with prices rising rapidly, shoppers (or "housewives" as they were known in the '70s) preferred the idea of an up-front discount, rather than waiting around and saving up stamps to redeem a few months later.
However, people rather took to the idea of the 'Green Shield redemption stores', later known as 'gift houses' and finally as Argos.
The idea of pre-selecting items from a nice catalogue and just turning up to the counter to pay and collect them seemed to strike a chord, rather than wandering around a number of stores.