Stop doing it and see if anyone notices. In case of Charged, Owen took a break for a couple of weeks and got flooded with questions. That's how he knew that someone out there was truly interested.
Owen was a journalist, addicted to Twitter, with a personal audience of 20k people. He began to worry that Twitter was going to go away, and he would lose his followers and not be able to be in touch. Charged seemed like a great way to provide value to his followers, while also writing regularly on the topics he found interesting.
Click-tracking software puts an invisible pixel at the end of the newsletter template, but Gmail will partially hide your email if the content is too long and require the user to click-through in order to download and see the rest of the message, therefore never triggering tracking for emails that were not fully opened. If you have a newsletter and are seeing very low click-through rates, this might be the case, and your solution is to redesign the template to fit within Google's limits.
Advertisers often look at lists sizes first, instead of looking at open rates for a newsletter. For example, they would be more willing to support someone with 50,000 subscribers than someone with 13,000, even if the latter gets 2-3x more people to actually open the newsletter. Size matters.
Kai, the founder of Offscreen, sells only a few spots for ads that feature only the logo an two lines of text for each advertiser. Slack always just says: "Hey, it's Slack. Want to try us? Use this code. If not, have a nice day!" It's a really refreshing take on ads in today's world.