This is my guess, based on the old system, to why some people cannot look up their application on ECAS:
Generally speaking CIC hasn't touch your application yet and thus have not "input" it onto ECAS, doesn't matter whether you use UCI or EXXXXXXXXX File Number. Once an officer has started your application then you will be able to look up your status on ECAS first using your UCI. Sometime there is a delay to link the file number to the ECAS system from personal experience.
At the moment I cannot look up my EE application on ECAS either but I can find my old PR application which is still in progress. So rest assure that if the call center agent can see your application then you don't have to worry. I phoned the call center agent a lot lately and ensure that my application is still there. One of them suggested that I submit a case specific inquiry form to ask why I cannot look up my application using ECAS and I did, but CIC won't respond to me until 35 business days in regards to the case specific inquiry.
Anyway, hope this helps.