I’ve noticed with with my campaigns that if I upload 2 different ads, one ad will be given all the impressions and the other one won’t even have a chance to be tested. This results in only ad being given a chance to perform. I’m using a CBO.
If you are using CBO, there is no way around this, when using CBO, facebook will always spend money on ad that facebook thinks will perform better either in terms of ctr or cpc. Let’s say you have two ads in a campaign using CBO, AD A and AD B. If AD B is receiving higher CTR then facebook will tend spend more money on AD B rather than AD A even if AD A is performing better for your metrics. The only way I think you can get total control of your ads by making single ad set campaigns, using one adset per campaign will give you much more control over your spending on ads.