did you probably choosed a "wildcard" - A - entry for your domain "YOUR-DOMAIN.COM" at your DNS - settings, which would certainly interfere with an additional ( separate ) "SUBDOMAIN.YOUR-DOMAIN.COM" A - entry?
Did you wait for the worldwide DNS - servers to be synced? ( This can take up to 24-72 hours, depending to your used nameservers! ).
Pls. note, that it is far easier for people willing to help you, if you step out of anonymity ( and provide the actual FQDN and the correct name of your subdomain ), so that we are able to start investigations.