It is being quite fussy as to which items it is letting you use it on, but def still works. What is it saying? If it says the code is invalid/expired then it is the code that isn't working. If it says discout applied or whatever and hasn't taken off any money than it is your item.
Or just try doing it as a new customer.
Best code is 2719 - £10 off no minimum spend! So you can get stuff for free/very cheap