Technical SEO mistakes on your website can have a serious adverse effect on its rankings. For example, your site should be well-optimised for mobile devices, meaning that each page should load fast and be user-friendly. They should also contain the appropriate use of header tags and relevant page meta titles together with a succinct meta description. Nowadays, all sites should also load over HTTPS, providing a secure browsing environment for your users.
There are numerous other factors and/or mistakes, errors and omissions that can affect your site’s performance, most of which can be fixed quite quickly – I’ll be able to look through all of these to identify any issues your site may have.