Problems with samba over VPN

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Hello , i have created a VPN  ( with openvpn-2.0_3  on freebsd 5.4 ) between my network who has a gateway on freebsd 5.4 and others computer on win xp, 
and a network who has windows xp home as gateway.Windows xp home used openvpn 2 too and i have created a bridge in my freebsd gateway and the windows xp gateway have created a bridge too.Now , i have configured samba on my freebsd gateway and i must see normally  (in my computers on windows xp ) the windows xp gateway of the other network in my workgroup but i just see this computer for ten minutes !!! After that , i don't see it any more and i don't understand why !!! Anyone have an idea for this problem ?