default gateway and dns unreacheable for ppp clients

hi
i provide VPN for windows clients. some of htem can connect to internet and some of them not.
in the computers that cant connect to internet after logging in with their vpn account (PPTP) they cant reach DNS server.

this is the firewall and dns and ppp settings:

Fire wall Setiings:

 0   chain=input action=drop protocol=tcp port=!80,8080,443,8291,1723,3389,53 

 1   chain=input action=drop protocol=udp port=!500,50,4500,1701,53 

 2   chain=forward action=drop protocol=tcp fragment=no 
     port=!80,443,23,8080,53,3389 connection-type=ftp 

 3   chain=forward action=drop protocol=udp port=!80,443,53 

Nat settings:
 0   chain=srcnat action=masquerade src-address=172.16.0.0

 1   chain=srcnat action=masquerade 

DNS Settings:

 servers: 172.16.0.1,207.182.147.129,207.182.147.162
  allow-remote-requests: yes
    max-udp-packet-size: 512
             cache-size: 204800KiB
          cache-max-ttl: 1w
             cache-used: 13KiB

PPP Profile:

name="PPP Profile" local-address=172.16.0.1 remote-address=External_Pool 
     use-ipv6=default use-mpls=default use-compression=yes use-vj-compression=yes 
     use-encryption=required only-one=yes change-tcp-mss=default rate-limit=128K/128K 
     dns-server=172.16.0.1

the public ip of external interface and default gateway is shown.
but for the clients it must be 172.16.0.1 and the clients ip rane is 172.16.0.0/16
the clients can get the 172.16.0.1 for their dns successfully but some of them cant resolve from DNS.
i can surf the websites whith one of my computers but on the other one cant (with the same account).

whats the problem?
Thanks

there is no solution for this problem?