Please consider answering my comment with a reply-comment. That's easier ;)
> It is likely that this is indeed the problem
Well, the pretest link looks fine. Should not be the problem, if you actually sent them this link. Please note that editing an existing email to update the link and send the changed link to another person would not be a good idea.
I have you started two pretests. Do you see the data and comments (only page 1) on your data?
Do you have the times when the others started the questionnaire? That's what I need to track the events in the server logs.
> collecting data from one person, I see that 14 questions are on one page and 19 on the other, while for another tester, all the questions are on the same page.
Usually, you decide on which pages to display which questions. Is there the same value in the QUESTNNR variable for both cases?
> one tester has SD01 but not the other two, the other does not have SD02
Also sounds like you have different questionnaires in your survey project ... see "Compose Questionnaire" -> "Manage Questionnaires". Is that the case?