I've checked the topcashback terms and they do state -
Applicable transactions for cashback can be defined as transactions that fulfil the following criteria:
(a) Transactions which are tracked directly back to your account by means of an identifying tag, as reported to us by the merchants and/or their tracking networks. Please note that you must be logged into our site at the time you click through to the merchant in order to have your transaction tracked by means of the identifying tag. We will however, take reasonable measures to warn you if you are clicking through to a merchant whilst not logged into the TopCashback website.
(b) You also must click through to the merchant via the main 'Continue' or 'Continue to Merchant' buttons on our site. If you click through via a sponsored advert on our website, then no commission or cashback will be earned. We will endeavour to make clear which links are advertising links and therefore will not be applicable links for earning cashback.
Did you delete your cookies and cache before placing the order? Did you follow the TCP route or did you have a browser running alongside which was linked to the BT site?
I can check what's gone wrong with this for you. Can you drop me over an email with your details? You'll get the contact the moderators link in my profile.