Marvin Controlled Campaigns
In Marvin controlled campaigns, the possible reason is that Marvin decreases the bids until it reaches the Target CPA that you have defined. Marvin avoids taking rigid changes as it requires statistical significance for such actions. Therefore, you see a gradual drop.
The competition within the country that you are targeting or the season in which you set the campaign might be too high.
There might be other campaigns overlapping with this campaign
There might a deduping scenario which is that you are targeting the same target group within different ad sets or campaigns