DHCPD, no free leases

Question

I have an ubuntu 14.04 server and isc-dhcpd-4.2.4. The clients that i already defined in config can get their ip addresses, and newly added mac addresses cannot get their ip number and i got "no free lease" error.

My /etc/dhcp/dhcpd.conf file is:

option domain-name "xxx.com";
option domain-name-servers 10.0.0.2;
option local-pac-server code 252 = text;
option local-pac-server "http://10.0.0.50/proxy.pac";
default-lease-time 28800;
max-lease-time 86400;
authoritative;
ddns-update-style none;
log-facility local3;
option ntp-servers 10.0.0.50;
option routers 10.0.0.1;
option subnet-mask 255.255.252.0;
option netbios-node-type 8;
option netbios-name-servers 10.0.0.2;
option netbios-dd-server 10.0.0.2;
option 46xxOptions code 176 = string;
option 46xxOptions "MCIPADD=10.0.0.92,MCPORT=1719,TFTPSRVR=10.0.0.40,VLANTEST=0";

subnet 10.0.0.0 netmask 255.255.252.0 {
        group ist {
                include "/etc/dhcp/dhcpd.body";
        }
}

there are 794 ip addresses defined in dhcpd.body, the last two lines are like that:

 host s793 {
    hardware ethernet 00:80:64:7B:D5:38;
    fixed-address 10.0.3.97;
 }
 host s794 {
    hardware ethernet 00:80:64:67:4D:42;
    fixed-address 10.0.3.98;
 }

my /var/lib/dhcp/dhcpd.lease file is:

# The format of this file is documented in the dhcpd.leases(5) manual page.
# This lease file was written by isc-dhcp-4.2.4

server-duid "\000\001\000\001\032\353\222k\000\025]\000\023\031";

But when i check the log:

tail -f /var/log/syslog

I got this errors:

Apr 28 13:50:34 okubuntuserver dhcpd: DHCPDISCOVER from 00:15:70:4d:2c:ff via eth0: network 10.0.0.0/22: no free leases
Apr 28 13:50:35 okubuntuserver dhcpd: DHCPDISCOVER from 00:15:70:62:db:76 via eth0: network 10.0.0.0/22: no free leases

But those mac addresses are already configures in dhcpd.body.