Here's how. Brief explanation by Nuffnang crew - Robb.
Allocations for Nuffnang ads are counted based on the number of impressions or clicks the advertiser purchase divided by the number of days the advertiser intends to run the campaign. Take for an example, an advertiser purchases 700 ad impressions on your blog for a week's period. That means, it will be divided into 7 days and each day the ad will only be served to the first 100 visitors to your blog.
As each day begins anew at 12.00am, the ads will start showing from that time till the allocation of 100 impressions are used up for that day. That's why at times, ads don't show anymore once it reaches the late evening due to the fact that the impressions purchased for the day has all been used up.
If anyone doesn't understand anything, it's totally fine. You are free to ask Nuffnang about their services and they'll be glad to assist with their helpdesk system. :)
P/S: Thanks to Robb for the explanation and clarification.
P/S: I Love Nuffnang has been changed to Simple Blogger Tips to accommodate more tricks and tips for Blogspot bloggers - no longer limited to the world of Nuffnang!
Stay tuned for more tips and tricks!