2nd of dual monitors not accessible

I have synergy server 1.4.8 running on Ubuntu 10.04. There are three clients: a Win7 laptop with an external monitor, a Mac Mini (normally in standby) sharing a monitor with the server system, and another Ubuntu 10.04 box with only one monitor.

Physical setup:
Laptop | Laptop monitor | * | Ubuntu Server main | Ubuntu Server extension | Ubuntu client
Where * is the sleeping Mac client.

Synergy setup:
Laptop | Mac | tron | Jump

I can move the cursor freely between the four monitors on the left if the Ubuntu client is not connected. Once the Ubuntu client connects, though, I can no longer access the Ubuntu Server extension. The mouse moves directly from Ubuntu Server main to Ubuntu client (and back.

Any ideas for how to fix this? I have copied the synergy.conf below. Thank you.

section: screens
    Laptop:
    Mac:
    tron:
    Jump:
end

section: aliases
end

section: links
    Laptop:
        right = Mac
    Mac:
        right = tron
        left = Laptop
    tron:
        right = Jump
        left = Mac
    Jump:
        left = tron
end

section: options
    screenSaverSync = true
end