You said "we use some affiliate links in the newsletter but they don't really bring in any revenue"
If your affiliate links aren't bringing in revenue what makes you think and ad will bring revenue to and advertiser?

You said you have a list of 2500 targeted people, what is the open rate for the newsletter and the click through rate, how many click on the links.

By Newsletter what do you mean? Is it an old school newsletter filled with a bunch of stuff. If so that could be part of the problem with the affiliates. Newsletters can be overwhelming and too much stuff for our information overload society.

Try sending one topic per email at one per week. 80% value & 20% promotion, meaning either 8 out of 10 emails provide value and only two promote something your's or someone else's, or 80% of each email is value and only 20% promotes something.

If your subscribers on YT are active & you videos have lots of views to the end, that may be a better place to look sponsors. Tim Schmoyer of Video Creators has some videos on how to do that. Also I would use the channel to continue to build the email list.

Guard you list and only present them things you are willing to back up 100%, if you guard them they will become your raving fans and will buy & promote for you. Continually put things in front of them to buy especially from other people and they will grow weary and move on.

Schedule a time if you'd like help deciphering your analytics or reviewing your content.