zotpress working randomly

Hi, I have used the zotpress shortcodes successfully but the displaying items works randomly. Sometimes it works and other times (more often) it says “Sorry, there’s no items to display.” On the plugin wordpress admin interface, a re-import always pulls in collection names but when trying to import items generates the error “Sorry, but there was a problem importing items: Internal Server Error”. Not quite sure what to try next. When it works, it’s fantastic. I know that all the parameters are correct in the shortcode as it works sometimes. Where should I start to track down what’s going on?

https://ww.wp.xz.cn/plugins/zotpress/