Readers that come from your newsletter are already converted to you. Readers that come from an advertisement do not know who you are and are evaluating your website to see if they want to convert to you. It is reasonable to expect that only 10% of these people want to convert to you. The other 90% after scrolling down your website are not interested in converting to you. Your website did not contain the hook to make them convert. You should improve your website.

TL;DR The adverts performed 100% in bringing visitors to your website. That is their job. Your website is not converting them to real users 90% of the time. Your website failed.