First define what “not converting” means
For a local service, success may be a suitable appointment. For B2B it may be a qualified sales conversation; for ecommerce, a confirmed order. Specify the outcome and check business records before judging a campaign from Ads Manager alone.
Use complete comparison periods and allow for the normal buying delay. No clicks, no website action, unsuitable leads and unprofitable purchases are separate diagnoses. If valuable conversions exist but cost too much, move to cost-per-acquisition diagnosis.