Slider Thumbnail not showing

See screenshot:
https://postimg.cc/G8z1SRDG

You have a lot of Mixed Content errors.
This means some of your image are being loaded under https:// even tho your website is being loaded over https://

There may be a setting at your host to “force https://” :
https://www.bluehost.com/help/article/force-ssl-on-all-pages
https://www.bluehost.com/resources/how-to-do-an-ssl-redirect/

Or, you can try this as well:
https://en-ca.www.ads-software.com/plugins/regenerate-thumbnails/

You can also search and replace all the https:// strings in your database your database to https://
Careful – there is no ‘undo’ in a db search/replace.
You can try this:
https://en-ca.www.ads-software.com/plugins/better-search-replace/
or this:
https://interconnectit.com/search-and-replace-for-wordpress-databases/