> at the moment three out of four people say they have replied and I don't have their data
That sounds like a systematic issue, true.
Could it be that you have sent these people a personalized links, so that they edited the previous interview, each? Have you tried the link that you sent them, yourself?
If you have a URL and somewhat accurate times, we can check the logfiles and see when the questionnaire was filled out and which CASEs these are.