More important, you need to quantify for the medium to large brands that your visitors are BUYING, because while 50,000 is an impressive number of hits, it means dink if you have no conversion metrics.
As far as selling it to them--if you have something of value to offer them, I found that polite persistence works over time. That might mean providing additional data and creating enticing offers that make sense (I don't know what you're charging them; determine whether your ad pricing is appropriate for that amount of traffic.) Create some intro specials for three to six months that include monthly e-mail blasts from you about the specials.
Last but not least--ask your contacts what they need to close a deal with you. It may be the feedback you need to tweak or fine-tune what you are doing, so you can achieve the ad revenue you are seeking. For more strategic marketing tips, visit Write-Communications
Wishing you success,