You need to look YoY the following metrics to have the total picture: Impressions, Imp. Share, CTR, Conversion Rate, Total Spend, Conversion Value, CTR, CR and finally ROAS. Your traffic has probably dropped and it's expected. Watch out for: 1) CR. If you have considerable drops in CR then that means that your visitors are more reluctant to purchase (as a result your CPAs will increase) and 2) Conversion value