This happens when a Sublime window is open but no tabs are open. The command-line switch -n should open up a new window, but it simply focuses the already-open window.
There are instances when I have a Sublime project open but no tabs open and I want to keep that window open, and open up a new Sublime project. I have “subl -n” bound to a keyboard shortcut to open up a new Sublime instance. What should happen: a new Sublime window opens up on the current workspace. Instead, nothing happens, since a Sublime window is already open. So the command line switch -n is not doing anything.