Increase your Bid Size
When you use the Auction Campaign buying type, the bid of your ad set can determine how competitive your Campaign is at winning impressions and spending money. The higher your bid is, the more competitive your Campaign will be. In this instance, it's more likely that you'll spend your budget.
If your Campaign is spending slower than expected and your audience reach is low, try increasing the bid of your Campaign from the Campaign Grid or increase the bid of your ad sets from the Ad Set Grid.
Keep in mind: If you're bidding on a CPA or an oCPM towards an action, you should bid the maximum you're prepared for that action to occur. If you don't have a conversion history of more than 20 conversions or a one day post click conversion rate higher than 0.5%, try reverting to bidding on a CPC to build up conversion history before using CPA or oCPM bidding types. Without a conversion history, Facebook cannot predict the CPA of your Campaign and allow your Campaign to spend.
Audience size too narrow
The size of your audience can determine the amount of budget you're able to spend. If your Campaign targeting is too narrow, it can prevent your Campaign from reaching enough people to spend the budget you have set. You can measure whether your Campaign's audience size is too low by using the Reach and Estimated Audience Size statistics (From the campaign grid, click the Choose Column to select these two statistics).
If you see that your reach is 80% or higher of your estimated audience size and your Campaign is spending slow, consider expanding your audience (Click into a Campaign, and then click Add new ad sets). You can then enter a new audience to target ads towards.