BM 3.8 SP2 on NW 6.5 SP2 and BM VPN Client 3.8.7 on W2k SP4

Having problems with VPN clients connecting to our servers. The VPN tunnel
connects with no problems but I cannot ping or log in to anything on the
network:

I can connect a VPN to our proxy server (default gateway) in backwards
compatibility mode and I CAN ping everything on the network.
I can connect a VPN to our proxy server (default gateway) in IPSEC mode
(NMAS) and CANNOT ping anything.
I can connect a VPN to another vpn server in backwards compatibility mode
but can only ping a machine with it's default gateway set to that server.
I can connect a VPN to another vpn server in IPSEC mode and CANNOT ping
anything, even the machine with it's default gateway set to the vpn
server.

I've checked everyhting about ten times and have got 2 differnet
consultants to check our settings with no success.

Any ideas ?