Here's what could be done to fix the issue:

Target an entire or multiple categories (if only a few ad units with no actual unique traffic are targeted)
Increase daily budget (if it's too low)
Increase max CPM (if it's too low)
Allow new locations in geo-targeting (if you're using the feature)
Add graphical banners (if you've enabled the option "Show banners only")
Uncheck "Alexa rank only" or/and "A-ADS approved sites only" if these settings are active

The simplest solution would be to create a new campaign (without specifying any settings) in a desired language and check how much traffic is available. Then adjust options one by one.

If nothing helps, don't hesitate and contact helpdesk.
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