The most common reason this happens is that you have one or more discounted items in your basket. To apply the 1st order discount, or any other, reward, you'll need to ensure your basket contains only full price items.
The other common reason a reward isn't applied is that you have more than one reward and our system has chosen the one you don't want to use on this occasion.
If this is the case, at your basket, simply click 'change' where indicated below, to use a different reward...
Once you've clicked 'change' you'll be presented with this screen where you can choose to use a different reward, or no reward at all if you'd like to save it to use on a future order.