Can't get my characters to fly in WOW

[Hexe]

My mapped key for 'Jump' is working fine for both characters, but if they are on a flying mount, all they do is hover for a couple of seconds and then touch back down. I am using the default Jump keymap that the wizard gives you. I can't seem to figure out what could be causing this.

Edit: also, on further testing, I've discovered that if they are already in the air, pressing the key won't make them fly higher either. Urg! So frustrating.

Any help would be greatly appreciated.