using Sync You have to imagine Your model more in terms of "surfaces belonging together" rather than a classic feature from traditional environment
And so, if You don't select the top plane face, the mirror "feature" can not create a full body on the other side.
To keep this in mind will help You a lot in future sync projects!
really a lot of helpers and tutorials will be found when using the Solid Edge help - Selecting the online tutorials and then You directly are at the Solid Edge training page where everthing could be found
Here is the link too:
There also are the tutorials for printing and self study which I like very much and can recomment too: