Either we got no JSON from the API. Or the basenode-parameter is not ok.
Switch on the Debugmode of the Plugin!
Switch to gutenberg-Blocks!
There the JSON Content Importer Block gives an easier way to use the JSON-APIs.
Or: Switch on the Debugmode by adding "debugmode=10" in the Shortcode.
If all is without success: Open ticket at wordpress.org please
EMPTY api-answer: No JSON received - is the API down? Check the URL you use in the shortcode!