can't access shared folder on domain server by name but I can by ip

I have server with windows server 2008 r2, router works as dhcp, after I created domain on it, and created users, making clients access the network via new users of domain then the computers work fine but after sometime some of them cant access folder of domain server except by using it's ip, sometime when I change the ip of the client then it's working fine but after a while the problem comes again. appreciate your help.