Many issues that you may run into such as: white screen, demo content fails when importing and other similar issues are all related to low PHP configuration limits. The solution is to increase the PHP limits. Check our requirements.
If you are not able to increase server limits you can try to switch temporarily to a default WP theme which is much simpler and switch back afterwords. Repeat the import process several times until you get the pages imported.
Don’t worry, media and the content already imported won’t be duplicated. Duplicated may be only menu items but you can quickly sort that out manually.