Ads Manager uses an auction system that means your ad is potentially competing with other ads for the same impressions. Some things that can impact the delivery of your ads are your maximum bid and targeting selection. Since Ads Manager uses an auction system, choosing too low of a bid may result in your ad not getting high rank in the auction or not winning the auction, as ads with higher bids will get prioritized delivery and placement. Selecting a smaller target audience may also make getting delivery harder. You can raise your maximum bid and broaden your targeting to help drive more delivery.