Examples of steps that might have an important negative impact in the available volume you will see for your campaign:

exchanges not active
incompatible operating system/no supported identifiers
incoming deal is not accepted by the campaign/campaign only runs on deals, but bid offer does not support deals
creative size/type incompatible
no location/location not compatible
publishers filtering (blacklist/whitelist of publishers)
campaign targeting (mobile operators, languages, user agents patterns, etc - common options available in the targeting section of any campaign)
retargeting: blocked by list

How to increase reach in your campaigns if you are not getting to your goals?