Responses are not being saved

Hi all,
I have been using Qualtrics for two years now and use it for several questionnaires.
I have noticed that for multiple of my questionnaires the data of some participants is not saved. I have contacted these participants to ask them at what date they completed the questionnaire but their data (based on their ID code) is nowhere to be found.
I have searched the "recorded responses" as well as the "responses in progress". In my settings I have indicated that incomplete responses have to be saved and that responses are considered incomplete after 6 months.
Is there some other option I could check? I reached out to the support of Qualtrics but they could not find anything that could explain the missing responses (e.g., the data of the respondents that are missing were not deleted).
Thank you for the help!