The only reason may be the use of a non-responsive theme. in 2021, every theme has a mobile responsive option available. So there might be the following reasons for this issue
1- You are using a non-responsive theme
2- Your theme has the option to optimize your site for mobile separately. But you have not optimized it
3- You have not saved the settings you made.
I would recommend you to use a high-quality premium theme that prevents you from this kind of situation.And it saves you a lot of time!
Thanks
Sibghat
