What decreases conversion rate?
One of the most common reasons for a drop in conversion rate is a change in traffic sources. For example, if you've started running ad campaigns on a new platform or targeting a different audience, it's possible that you're not getting as high-quality traffic as you were before.
What affects CVR?
CVR tells app advertisers how well their ad converted users. However, the act of 'conversion' varies depending on the pricing model - in cost per install campaigns, for example, conversion is measured by app installs, while in cost per engagement campaigns, conversion is measured by engagement events.