Replication timeout from external source

We have a few external users replicating their Notes client to our Domino server over the Internet. No problems at all receiving mail, but some of them have problems replicating large documents to the Domino server. It doesn’t always happen, a document that won’t go through one time will go through another. There doesn’t seem to be a common network configuration problem, some of them are behind routers/firewalls, etc. at home, others are connected straight into their cable/DSL modems. The problem occurs at all times of the day, whether the network the Domino server is on is busy or not.

Is there a log somewhere I can look at that would maybe provide more information as to what’s going on?

Thanks, and please let me know if you need more information.

Jason