Question : Cant ping FQDN for some of the machines but I am able to ping them by IP

Hello,
We have some of the computer that could not be pinged by FQDN, I got Ping request could not find host FQDN of server. Please check the name and try again. I only can ping it by IP
Have tried to restart the machine it did not make any difference, the machine is getting the right configuration for the DHCB, DNS.
Any ideas?

Thanks in advanced

Answer : Cant ping FQDN for some of the machines but I am able to ping them by IP

Yes I did and the DHCP was releasing the IP address fine, we asked the users to restart their computer to get an IP from the new DHCP (we have disables the old dhcp in the main time) some of the machines were fine and some others we could n't ping them with FQDN after we tried to manually resister the DNS for some of the computers and restart the rest it was fine.
Thanks you all for your help!