It's hard to say. I would suggest
the following (on both ends):
clear counters
start pinging (from either end)
sh interface - confirm that there are
no drops or errors after a ping failure
If you are using MPPP or any other
feature, try disabling it to see if that
makes a difference.
If the interfaces are clean, see if you
can determine if the packets are being
received and forwarded at either end.
You can try this with "debug ip packet
detail" on either end (careful, this will
be a lot of output), or with an analyzer
at either LAN end.