A common source of confusion and frustration is the unfilled impression. You’ve got a bunch of ads setup, so how can you be getting blanks?
One reason is the failure to duplicate creatives. The way that DFP works means that each creative will only be used once per ad request.
Suppose you’ve got a page with two ads. Both are sized 300×250 and appear in the sidebar. One at the top, and one at the bottom.
You setup a new line item in DFP and target both ad units, so far so good.
Now you upload a single creative. Assuming that AdSense dynamic allocation is disabled and there are no other eligible ads running, what will happen?
DFP will show one ad, and the other slot will be blank. This will count as an unfilled impression in the reports.
Because there is only one creative, DFP can only show it once!
The solution here is simple, you need two creatives. The quick way to do it is to navigate to the Creatives tab, under that Line Item. Then click the checkbox for each creative you want to duplicate and choose “Copy creative” from the “More actions” box. If you do reporting down to the individual creative level, you might want to change the name here as well.
Now an ad will appear in each slot and you won’t get blanks.
So how many times do you need to duplicate each creative?
Basically, you need to have one creative for each ad slot size. So if you’ve got three 300×250 units, you will need a total of 3 creatives to fill them all without blanks. When in doubt, it doesn’t hurt to over-duplicate a little bit. DFP will just rotate them. But don’t go too crazy.