Hi Lilian
Thanks for reaching out in the Lucidchart Community! Is the page that you are trying to export a Master Page? If so it will be omitted from a downloaded version of your document and you will need to convert it into a page before exporting. To do this left click on your Master Page tab and select "Convert to Page".
If that’s not the case issues with exporting documents are often browser or network related. Could you try exporting your document in a private or incognito window? This will clear your cache and cookies and disable any extensions that could be interfering with your account. Could you also try the export again on another network ensuring that you are on a strong stable connection?
To learn more about how to successfully export your Lucidchart documents please take a look at our Export Your Lucidchart Documents article from the Lucidchart Help Center.
Hope this helps! Please let me know if the issue persists.