Can VPN but not access any machines

Hi all,

I'm having a problem where I can connect to a client's VPN 100% of the
time, but can only RDP into or ping their local machines maybe 30% of
the time (it seems to have periods where it's working and then periods
where it's not working). I'm not after so much a solution, because I
realise that this is nowhere near enough information for someone to
even begin to try to figure out what's wrong (although if you have a
solution that would be fine ), but more tips on how/where to even
START looking.

Also, we VPN into a number of other clients' machines, and both the
VPN and being able to RDP into their machines works 100% of the time
(we are experiencing problems with only one client).

Any suggestions would be much appreciated.

Taras