Sadly, that's not it. I use a separate install for these purchases to make sure none of my extensions or settings on my main browser interfere. Yesterday I had the Ticketmaster purchase of Monty Python tickets track, but the John Lewis and White Company orders immediately after has not. Those were both for one item each and processed quickly, so it's not even a case of the tracking 'timing out' as far as I can figure.

I can't think of much else to try, but I'm all ears for suggestions!