There are some errors in the site template on the page with Popup – https://prnt.sc/hh7bua – you can also see it in the dev’s tool at the Console Tab.
These styles don’t load, because in their code the load goes through http, but your site explores https.
This may affect your other plugins and Popup by Supsystic.
Please contact your site template developer to fix it.
We also will look through your issue more detailed.