Third level menu item is displayed next to the 2nd level menu item instead of under it

I use module superfish version 7.x-1.9.
I've created a first level menu item (FIRST).
Under FIRST, I've created a second level menu item (SECOND).
Under SECOND, I've created a third level menu item (THIRD).

The result :
The SECOND level is a child of the FIRST level, which is exactly what I expected.
The THIRD level should be a child of the SECOND level, but it's not. THIRD appears next to SECOND ,instead of under it.

Click here to watch a very short video that shows the problem.

Any idea how I can fix it ?

Thanks in advance.