An advertiser enables target cost-per-acquisition (CPA) bidding and notices that conversions decrease. What might cause this?

An advertiser enables target cost-per-acquisition (CPA) bidding and notices that conversions decrease. What might cause this?

The target CPA bid was higher than the recommended amount
The target CPA bid was lower than the recommended amount
The conversion tracking code snippet was not added to the site

The cost-per-click (CPC) bid was lower than the recommended amount