If you have tried to import a demo and received message: Demo is not Fully Imported! Then try doing the following:
1. Install & activate all required plugins for the demo you are importing in STEP 1:
2. Check your server settings and PHP Configuration Limits follow this:
You can do this on your own, or contact your web host and ask them to increase those limits to a minimum as follows:
Follow this to know How To Increasing Memory Limit To PHP
3. Reset your website and try import the demo again.
It is recommended uploading sample data on a fresh WordPress install to prevent conflicts with your current content. You can follow this to know How to Reset a WordPress Website with plugin: WP Reset.