Hi @charlottebyrd!
The cause of the problem is most probably a conflict with another plugin.
Could you please, firstly deactivate all your other plugins, except Slider WD and see whether the thumbnails will appear.
If the problem will not be occurring, please activate plugins one by one and check Slider WD simultaneous, to find which one is causing the issue.
In case the issue will appear while all plugins are deactivated please contact our support team using this form (select “I have a pre-sales question” and the form will appear).
Please, mention the URL of this forum topic in your message. We will provide a solution as soon as possible and share it here as well.
Thanks. Have a nice day
