Request ip address to the DHCP server in vpn

[illuminate]

Hello, recently i manage to create a vpn server by following this tutorial
http://blog.lincoln.hk/blog/2013/03/19/ ... er-on-vps/

and connecting by using local bridge as suggested for better connection performance
http://blog.lincoln.hk/blog/2013/05/17/ ... al-bridge/

My server have total of 14 user. However sometimes, some the of the user claim that they have to take a long time to request ip address to the DHCP server, and the dialog box just disappear, which is close without any error.

But the ip on their computer are still the same, by checking on the whatismyip website.. However i check on the virtual hub session, that their id does connected successfully.

The user said this problem just occurs sometimes. Others, it just works fine.
Is it because it try to request the same ip address that currently being use for another user or what?

Any help would be really appreciate.

Edit: problem similar
http://www.vpnusers.com/viewtopic.php?p=9480
but cant find the solution.

[moatazelmasry]

Maybe you have some bad DHCP configurations. What happens if you use SecureNAT and use the built in DHCP server. Does your problem go away?