Could be a few things that cause this...
1. You are not at the epo server, when you are trying this.
2. your Agent Policy has this feature disabled, or point #1 is set in the policy to only allow connections from the epo server (the SPIPE ID ip in the Sitelist only).
3. Your Sitelist.xml on the client machine has the wrong IP for the ePO Server (would only apply if #2 is enabled).
4. Your not using Port 8081 as the wakeup call port. Check this in your server settings.
5. The Agent was installed as standalone (not epo managed), this is unlikely but I put it in the list as I have seen this before.
6. DNS/IP/Other Network issues - So this is a huge topic, but I would start with a NSLOOKUP machinename , then a NSLOOKUP <ipaddress> to ensure you are hitting the correct IP from the browser.
7. Hips/Firewall / Windows Firewall / Port Filtering on the Routers ets. (#6 sort of covers this).