Thanks Ram,
synthetic backups never gets finished successfully.
Here is the current firewall rules for the master server
IN
22 (tcp) ssh <- IN
161 (udp) snmp <- IN
835 (tcp/udp) YP <- IN
13724 (tcp/udp) vnetd <- IN
1556 (tcp/udp) PBX <- IN
13 (tcp/udp) Daytime <- IN
3001-3004 & 3101-3104 (tcp/udp) SM <- IN
OUT
53 (udp) DNS -> OUT
123 (udp) time -> OUT
25 (tcp) smtp -> OUT
514 (tcp/udp) rsyslog-> OUT
704 (tcp/udp) -> YP -> OUT
111 (tcp/udp) portmapper -> OUT
300 (tcp/udp) mountd -> OUT
302 (tcp/udp) statd -> OUT
304 (tcp/udp) nlockmgr -> OUT
875 (tcp/udp) rquota -> OUT
2049 (tcp/udp) NFS -> OUT
389 (tcp/udp) LDAP -> OUT
13724 (tcp/udp) vnetd -> OUT
1556 (tcp/udp) PBX -> OUT