Missing embedded data from a single response (collected from e-mail distribution)

Hi,
I have a few surveys I send out every week to the clients who pass a certain milestone the previous week. I use e-mail distributions to do that, with uploaded CSV contact lists.
Last week a new response was submitted with none of the embedded data from the contact list attached to it. (The distribution channel field was also marked as "anonymous," rather than "email" as usual.) Based on the latitude and longitude, I was able to figure out who had submitted it, so I just filled in the missing embedded data. But this week she got a reminder email, submitted another response, and e-mailed me to inquire why she'd gotten a reminder when she submitted last week. Sure enough, there's another recorded response with no embedded data at all, distribution channel=anonymous, with lat+long that matched her location. (I've now opted her out of the survey so she won't get another reminder.)
Any theories on why this one recipient's responses weren't linked to her embedded data from the contact list--and why this happened twice? I can't figure out what went wrong with her survey link, nor how to prevent it from happening again.