1) Creative: When ads are engaging and drive more clicks & installs, they get prioritized and run faster
2) Bid: Low bids can lead to under delivery. Advertisers should always bid their true max bid.
3) Campaign Schedule: Running your campaign ASAP tends to deliver faster than custom start & end dates
4) Micro-targeting: Super small addressable audiences can lead to under delivery
5) Increased Competition For Certain Users: Sometimes highly sought after target groups become more expensive for a period of time, causing under delivery
6) Seasonality: When many advertisers simultaneously spend large budgets at the end of a month, a quarter, or during a holiday period, prices tend to rise sharply, causing under delivery
7) Shocks to the system: Sometimes, advertisers spend a lot of money on FB ads over a short period of time. As a result, an advertiser who may have been getting normal delivery may suddenly see its ads stop delivering. For example, if advertiser A spends millions of dollars launching a product to a very specific audience during the course of a week, the clearing price for these users will obviously go up. Advertiser B who is targeting the same users may suddenly find its ads are seeing lower delivery.