Facebook shows only the one or two highest-performing ads in a campaign (that is, the ads with the highest click-through rates); the other ads in the campaign will not receive impressions. Therefore, if two ads are receiving impressions, and the click-through rate for one of them drops because of audience saturation over the course of the day or over time, that ad may no longer be shown, leaving just one ad. In turn, if the click-through rate for the remaining ad drops, or the bid is too low, that ad may no longer be shown.
NOTE: Facebook partially resets each ad's history each day. If an ad does not perform one day, it will have the chance be run again the next day, but Facebook will give it less of a chance than it will to better-performing ads.