FW-1 has been known to have these problems in site to site configurations so you are not alone. A good site for FW-1 info that covers issues such as the one you are having is Phoneboy (don't ask me why they chose that name) http://www.phoneboy.com/. I have heard that the simplest way to fix the problem is to create an artificial heartbeat between two devices on either side of the connection. If you have two windows machines it's real easy, just put them in the same workgroup; netbios will handle the rest (it's very chatty). If you don't want to do that, you can configure FW-1 properly to fix the problem. You'll have to check the site for the exact steps.
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