If you’re running a CPM campaign, this most likely means that your bid is too low for the audience that you’re targeting. You can increase your bid or change your targeting to win more impressions.
If you’re running a CPC campaign, either your bid is too low or your ad’s click-through rate is no longer competitive (our auction takes click-through-rate into account when serving CPC ads). We’d recommend starting a new ad and testing out different targeting and ad images to increase your click-through rate. You can find additional tips on increasing the click-through-rate of your ad campaigns here.