VPN - problem accessing network work drives on corporate network after establishing vpn connection.

We are using a cisco client (ver vpn to access our corporate network.  For some reason when users connect to the corporate network through the vpn they sometimes can not access the network drives or email.  The strange thing is some users can access all network drives from their cable modems from home, but not from hotels or vise versa.  There is never a problem establishing the vpn connection (this always works in all cases). Once they are connected they just can't access the corp network.
How is it possible that one minute they can access the drive and other times they can't?  Is it the vpn client software?  We have tried wireless & and wired connections as well as broadband cards.  It's normally a hit or miss!  If this is not clear please let me know....
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Rob WilliamsCommented:
Is there any chance the Corporate site is using a common subnet like 192.168.0.x, 192.168.1.x, 10.0.0.x?
If the office subnet and client's local subnet are the same you can experience this problem
That is frequently caused by a router somewhere that has it's MTU packet size set too low.
They call it a Black Hole Router.

Microsoft has a article explaining this here:

Here is a another good explanation from Cisco.
They have a link to a utility called Dr TCP that will help configure your Network card and set the MTU setting.

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