I've got a closed-access tokenised survey. 99% of the time everything is ok but sometimes people fill it in, we get an email with the responses but all the answers are blank. The tokens section shows the survey as being completed but no response is listed for the person.

Are there any ideas as to why the results aren't saved sometimes?

How is the survey displayed? Because a completely blank survey can only happen if you show all questions at once. Because the results are saved to the database when the respondent moves to the next page.

Did you look if there is another response with the same token? Depending on your configuration respondents can start a survey with the token more than once, until the survey is completed.

It's setup to display questions in groups over a few pages.

I've checked and this token is unique. Some people do get a survey for themselves and another for their partner - although they have different tokens would this be a potential cause of the issue?

How can I check that the respondents can only use the token once?

This sounds really strange. Because when moving from page to page the results are saved in the database. What can happen, if people don't finish is that the response is not marked as complete, but the results should be still there. As it seems to work with others, this is a really strange issue.

What version do you use? 2.5 or 2.06 or something older?

Regarding your other question - have a look in the manual here:
manual.limesurvey.org/Survey_settings#Tokens

Interesting for you should be:
-Allow multiple responses or update responses with one token?
-Enable token-based response persistence:

So to make sure respondents can only respond once, the first one should be set to "no", the second one to "yes".

Enable token-based response persistence means, that if someone started the survey, but stopped, then clicks again on the token link, will get back to the last saved state of the questionnaire.

I'll take a look at the link thanks.

I'm using Version 2.50+ Build 160415 although I'll be updating it later today as I see there's a security update. I've tried to recreate the problem several times but not suceeded so I suspect it's a very specific set of circumstances (or possibly user error).

One possibility is that they have clicked on the "clean and exit" button, which deletes all answers in the questionnaire from the database. Do you show this button in your template?

Yes it does. I've removed the button as it's not required and I guess we'll have to see what happens.

Thanks for the help.