L2TP/IPsec multiple client connections issue

Hello, i have problem with vpn l2tp/ipsec when more than two clients connect vpn from same network to the same remote network at the same time, 1 client at time can work. The other one cannot reach the remote network. One client at a time can work for 30s, then another client starts to work for 30s, then another and so on.
Any solutions?

Have a read here: http://forum.mikrotik.com/t/multiple-road-warrior-l2tp-ipsec-clients-behind-nat-solved/118206/1