The CSR, Key & Certificate share the same modulus. 

Find the hash of modulus of private key:
    openssl rsa -noout -modulus -in /config/httpd/conf/ssl.key/server.key | openssl md5
Find the hash of modulus of certificate is being imported but not getting imported:
    openssl x509 -noout -modulus -in /var/tmp/certificate.crt | openssl md5
Find the hash of modulus of CSR:
    openssl req -noout -modulus -in /var/tmp/server.csr | openssl md5

All 3 should match for a successful import to happen. If it doesn't reach out to the CA to get it signed again or generate an new CSR and get that signed again.