Excel data analyser

Hi, I am trying to use the Excel Data Analyser to present results of a survey with KoBo. I have followed all the steps in the manual. After uploading the 3 sheets and configurating everything the list of questions appears but when I select any of them, the chart is blank. I have read that this might be because some data are unreadable, but I have checked and everything seems to be OK. Could someone give me some light? I have been conducting more than 450 questionnaries and if I do not manage to present results in a proper way (with the standard data report format from Kobo is a nightmare) I would have lost 3 weeks of work ยก Thanks in advance