If you don’t indicate the itemId or the user of the item to filter it Tiki will consider your are ADDING an item not EDITING an existing one.
You can use an itemId parameter in the URL that go to your edit page: editingPage?itemId
I know there are "filters" parameters in the plugin trackers but I never used them.
My understanding is that you can use them so a a "User" will see his item edited. (not sure it will pref-fill).
On personal note I prefer to use the plugin list with an editing modal... but it requires advanced knowledge. ;-)