Here is a couple things that I figured out by poking around while using a schematic (11.1) . First when I have a symbol on a schematic and I select that symbol then right click, select "Symbol" and then "Edit Symbol". This brings up the symbol editor. Then I make my changes and save the changes and then close the symbol editor. The software returns me to the schematic but the symbol has not changed and it does not ask me if I want to change it or tell me how to change it. What I found out is that if you click on the schematic page (anywhere as far as I know) it will then bring up a dialog box that asks you if you want to update the symbol.
The second thing I found out is sometimes if I want to rename a net by selecting it and then select "Edit" and "Rename" , the "Rename Selected Net" option is grayed out and not usable. The reason this happens is that the "Select the line segment" option is selected in the "Options" window instead of "Select the entire branch" option. This is true even if the net is only one segment long. It would be great if this was improved.
I also faced the same problem as you did, so I can tell you simething.
After you edited the symbol, you then should firstly return to the Design window and left click the VHDL file of the symbol you have edited, then run Create Schematic Symbol in the processing window such that your symbol will be recreated. Then update it in the schematic edit window as you done before.