Thanks for the detailed reply, lots of good detail there to mull over... very interesting.
We hadn't realized that we should be relying on your thank-you page in order for the Ecommerce tracking to work. It's not explicitly mentioned in your Tracking help page. But that might explain a lot about what is happening, possibly including why we had, in the past, even using our own on-site thank-you page, registered some conversions.
As I go back through the logs, I realize that all the conversions we have ever registered have been for DOWNLOADABLE PRODUCTS. So, that implies that, at some point, those purchasers, even after having been re-directed to our thank-you page, would have visited the e-junkie file download page to get their product. Other purchasers, who bought physical items, would ONLY have seen our thank-you page and never ended up on the e-junkie site.
So that leads to my next question: was the ecommerce tracking code at some point included on the download page that purchasers receive in their purchase e-mail link? And was that removed, or is it still there? Or is that download link page in fact the same as the e-junkie default thank-you page?
I'm not entirely discounting the possibility that the AdSense modification might be causing the problem, but we hadn't changed that recently, and we saw plenty of conversions appear even after we inserted it. I'll revisit that code snippet if I don't get anywhere with the thank-you page experiments.
For now, I've removed the redirect to our on-site thank-you page and left the e-junkie default, and made some test purchases to see if they will register. I imagine it will take a couple days to see if they show up. I feel like we're making some headway on the problem now, thank you!