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How long have you been running this campaign? 18 conversions per month is not enough to use target CPA yet. I would switch to max conversions with a lower daily budget and let your campaign run for several weeks to build up conversion data and only after 1 ~ 2 months when you have had dozens of conversions, re-apply the CPA target.
Probably it has not enough data yet and is only focussing on the most bottom of the funnel customers to meet your CPA targets.
it could also likely be not enough search volume + too many competition on the phrase match keywords you are running.
