Windows 2008 R2 and TMG 2010 does not allow more than 2 VPN connections from the same IP client

Helloforum.
I'm having aproblem with myW2008R2 andTMG 2010.I havean office with5 clients,the office hasfixedinternetIP200.xxx
The problemhappens whenthe5 clientswant to connectto theVPN, more than 2concurrentCAN NOTconnect.
can you help mesolve the problem?

thanks
Greetings.

Costa Gustavo ACA - TI