Have you checked event log for any errors?
Are you trying to export pages from v7 and then import to v8, or within the same version?
Please note that export import is not recommended across different versions. It works in many scenarios, but might be issue in your case.
In case of importing same Kentico version pages. Was there any issue during upgrade? I would recommend to download clean copy of Kentico and compare file system as well as db. This way you can find some potential differences caused by faulty upgrade.