Ads not showing up depending on browser

Uploaded a new ad – and it shows up sometimes, I am on Chrome, it shows up, opened IE, not showing up, but there is a space for the ad, firefox same problem.

I love this plugin, and use it on a few sites, I hope you keep it current!

site is npva.org, the ad is NBFX

Thanks for any help!!

http://wordpress.org/extend/plugins/ad-buttons/