Why can the certificate not be found by the Ingress Controller, while the certificate has been issued? kubectl logs nginx-ingress-controller-X -n nginx-ingress

W1222 06:52:04.575115       8 controller.go:1058] Error getting SSL
certificate "app/app-tls": local SSL certificate app/app-tls was not
found. Using default certificate
W1222 06:52:09.955475       8 backend_ssl.go:48] Error obtaining X.509
certificate: no object matching key "app/app-tls" in local store


kubectl describe certificates nexus-tls -n cert-manager

Message:               Certificate issued successfully
Reason:                CertIssued


Discussion

It is unclear to me where to deploy the certmanager. I have deployed in a separate namespace, the cert has been created successfully, but the certificate cannot be found.
https://github.com/jetstack/cert-manager/issues/482#issuecomment-449664935


  Once I followed this
  https://github.com/jetstack/cert-manager/blob/master/docs/tutorials/quick-start/index.rst#step-6---configure-lets-encrypt-issuer
  and deployed the issuers in the namespace of the app, it works