Here's some general information on the key factors that are going to limit a campaign's initial traffic:
1) Available Impressions: Is there any traffic available in your target geo? Use the Inventory Tool to make sure the targeting you wish to have for your campaign has enough traffic available for the exchange you are using. For example, if you're targeting Smaato/South Africa/Creative size 300x50 - you only have 14,000 impressions available maximum, but you can see that by simply adding a 320x50 size creative it brings you up to over 5 million impressions. You should always be using this tool before you set up a campaign.
2) Placement Targeting: On any campaigns that you are targeting specific placements, remove these whitelists to increase traffic. We advise against targeting individual placements when you are initially testing because 1) you will be significantly limiting traffic and 2) if you are using placement names trying to category-target similar pubs, this is not necessarily the best strategy. More often than not, you may see that your offer converts on seemingly unrelated apps/sites at least as well (if not better) than compared to seemingly similar placements.
3) Session Depth: This is a very restricting targeting option. Pulling this is going to open up lots more traffic. It is a great feature to limit traffic, and potentially access some of the higher value impressions in heavily trafficked Geos. However, if you are targeting limited Carriers, OSs, etc in lower-volume countries, keep in mind that this can sometimes be more of a traffic limiter than a useful tool.
4) Daily Budget per Placement Cap: Again, this tool can limit traffic. It can be a great feature when testing a new campaign, but we recommend that you set it to at least 2-3 times your CPA to ensure you are not limiting traffic too much.
5) Frequency Cap: This is limiting how many times a given user sees your ads. Increasing the frequency cap will give you access to more impressions, but keep in mind that the more impressions you have with the same user, the less likely they are to interact with your ad.
6) Carriers: Whitelisting carriers is going to limit traffic. We understand that you may only want to target certain Carriers/ISPs but keep in mind that this can significantly limit the total available inventory.
7) Devices: Whitelisting certain devices will again limit traffic. We understand that you may only want to target certain devices but keep in mind that this can significantly limit the total available inventory.
8) Creatives: The main sizes are 320x50 (that account for 80%+ of traffic), 300x250, and 320x480. These are the sizes that are going to give you access to approximately 95% of traffic in almost all Geos, and even more in some cases. We strongly suggest you have at least one of each of these sizes in all campaigns.
9) Bid: This is critical if you are not using the auto-bidder. If you do choose to run campaigns with a set CPM (or for traffic, you override auto-bid with a flat CPM) you are going to want to look at the win rate. The win rate tells you how many bids you are making relative to how many you are winning. If your win rate is very low, you are going to want to increase your bid to win more traffic.
Now obviously each campaign will be different, so these factors are where you should start to investigate/reference if you are asking why your campaign is not getting traffic.