The import of the JSON file says 100% but then nothing displays?
Do you have the plugin WordFence installed?
If so, that might be the cause of the JSON failing to import completely as WordFences' Firewall blocks the file.
The best option would be to temporarily disable WordFence and then try to upload the JSON to confirm if that's the cause.
If it is, then you'll need to try putting the WordFence Firewall into "Learning Mode" so that it can automatically apply the correct white listing settings. You can read more about how to do that here: https://www.wordfence.com/help/firewall/learning-mode/
If you do not have WordFence installed and the JSON file still does not completely import, please check with your hosting provider if they have set any restrictions on uploading JSON files.
You're welcome to open a support ticket if you've tried the above and you're still experiencing issues.