I had this happen with mine mate. Tried hitting it with an Allen key, didn't fix it.
Then tried removing the nipple and greasing the o-ring at the back, which worked for a bit but then the same happened again with it.
In the end I just got Yankee Andy to swap the nipple for a new one (back at STB in September) and haven't had a problem since with it
The nipples that come with the X-streams do appear a little on the cheap side if you ask me though, other than that a bang on air system