If your theme is not responding to mobile devices then it could be an issue with your cache settings. Or the plugin itself.

Novapress Pro is definitely responsive.

I have not tested the theme with the WP Fastest Cache plugin, so I can not guarantee 100% compatibility.

My other recommendation to clear you cache manually after you have activated the plugin. I hope this will solve your problem.

If this does not solve the issue for you, I sadly don’t have a solution for you at the moment.

I might be able to do some basic testing with WP Fastest Cache in the future, but I can not give any guarantees.