I encountered the same for my purchase on 30 Nov, cashback not tracked accurately. Wrote in and this was the CSO's reply:
There are quite a few reasons why these orders may not be attributed to ShopBack. This includes:
1. If a user clicks on another chrome extension after clicking from ShopBack
2. If other web pages (including search sites, social media, store sites, promotional/discount sites) or web browsers are opened after clicking through ShopBack
3. Clicking through to another page from a banner, pop-up, or advertisement on the store’s webpage whilst shopping
4. Antivirus/Cleanup or Adblocker applications block or disrupt Cashback tracking (Tracking cookies are required to track transaction)
LL accept their explanation bcos point no.4 is valid for me but how true is it, I will never know.