Check that your head is screwed on snug. (On the vivi, not on your neck.

) Make sure that the metal ring inside the rubber stopper thing that sits on top of the head is flat side facing down. Just in case, go ahead and check that o rings (are there o rings? I think so , but I'm not sure) are in place and threads are properly threaded.
Did you refill it or was this the first fill? I notice that often when I am unscrewing the top to refill, the head turns with the cap. Watch out for this, and make sure to screw the head back into place before filling.