This maybe due to several reasons, please check the following:
- Missing featured image:
Make sure that all your posts assigned to this slider have a featured image.
To know how to assign a featured image to your post, please check this post.
- Try to deactivate all your plugins, if this works then please reactivate them one by one until you find which one is causing this issue.
- Deactivate all your widgets (especially if you have HTML/Text widget) and check if this resolves your issue.
- Stop calling any external scripts from “Header Code” area (Theme Settings >> General Settings >> Header Code) and check if this also resolves your problem.
In case you still have this issue, then please open a new support ticket and send us your FTP and WordPress login details so our support team can check your issue.