There may be several reasons:
Disapproved Ads: Your ads might be disapproved due to not complying with FB's advertising policies. One of the reasons might be the text overlay policy. If your ad has more than 20% text, then it will probably not be approved. You can check whether your ad will be approved based on the percentage of text on your creatives from this link in the Facebook Help Center.
Low Reach: Your campaign may not be spending due to low reach. If your audience size is not large enough and it is not scaling, you should review your audience. FB's best practice is to have at least 100K audience to never experience the low reach problem.
Low Bids: Campaign may not be delivering because of low bids. Your bid may be too low to win auctions. If you are certain that the campaign has enough reach and right creative, but it is not getting enough impressions, you should consider increasing your "tCPA" if you are using Marvin.
Overlapping Audience: Campaign may not be delivering due to overlapping audience. It applies if you have more than one campaign with similar targeting. Campaigns can’t spend their daily budgets because they compete against each other. If this is the case, we suggest you to make sure not showing your ads to the same users through different ad sets by using "Exclusions".