We've checked several times and don't see such an issue.

Please watch this video: https://www.screencast.com/t/PcKHNV9qL0m

We just see a small CSS conflict (it's shown in the video). 

Please use the CSS code below to solve it: 

#wpdcom .wpd-form-col-right .wpd-field-group.wpd-field-single .wpd-field-group-title{
     height: auto;
}

Put the code in the "Custom CSS code" textarea, located in the Dashboard > wpDiscuz > Settings > Styles & Colors admin page. 

Don't forget to delete the caches before checking. 

Sometimes, the comment form don't appear and all "Reply" button within each comment are inactive. After forcing the refresh (CTRL + F5), everything is OK.

Any idea ?

Regards