Facebook runs an algorithm to decide the quality score between your own campaigns. You could say that Facebook runs an optimization process for you. When you launch a campaign with many ads, Facebook give all of them the same exposure in order to identify the CTR of every individual ad. Once Facebook realizes which adds generate a higher CTR – or have a higher quality score – these get more impressions compared to the less converting ones. Over time, Facebook will stop showing ads with a lower CTR completely.
Facebook does this quick optimization work for you automatically, which can be great for conversion rates and something you can leverage a lot, especially if you have many versions of your ads targeting different demographics inside one campaign. However, in some cases it might work against your optimization work. Facebook tend to make their decisions very quickly and sometimes you might want to give your ads more time before the algorithm shuts them down.
In that case, create more campaigns with fewer ads inside each campaign. This will essentially force Facebook to show all the ads during a longer period before giving them a quality score and lower/increase the exposure of the ads.