0. Why are you still using the older TSU9400 software. Why not PEP2?
1. You should make sure the firmware in the extender is supported by the TSU9400 firmware you are running. Have you updated firmwares to match?
2. Did you try using Fixed IP addresses for the Extender within the Pronto Config itself? - Just because you set a fixed IP address on the extender does not mean Pronto uses that - Pronto will still try to discover extenders.
3. If you try to send an IR command to Port 1 of the extender, does the IR port light up?