Hi Adrian,

There can be many reasons as to why this happens. You will probably need to examine what the actual differences are and based on that, see if it's a general issue or it's just a CSS issue (e.g. different browser/window size etc.)

As an example: If the frontend code for your site uses JavaScript to detect specific browsers (e.g. mobile browsers) and serves different CSS to those, you will see a different result in Browserstack because it uses the actual devices to render the page, whereas the "mobile" view in the Umbraco Previewer just uses a narrow viewport (<iframe> I think).

Do you have screenshots that shows the differences?

/Chriztian