Understanding Ad Network Offer Criteria for Better Engagement

What criteria are used to determine which ads are offered to us on the ad network? Are factors like open rate or number of subs taken into account? We've been using the ad network for over six months, and despite growing from 300 to 12k subscribers, we’re still getting offers from the same advertisers. Our audience seems disengaged with these repeat ads, so we’re curious about how to access opportunities to new advertisers.