Can't take mage portals, others are fine

Trying to take a mage portal to Dalaran for example does nothing. Portals in cities e.g. Shattrah or Dalaran work or the Blasted Lands portal also works, this occurs on multiple characters (only have 2 I play on atm). I did take a portal very early on to Dalaran but that was months ago.

You have to be in group with the mage to use his portal. And you can't use it after the mage has entered his portal. Other than that, there should not be any problems with portals.