Since there has been a /report about it I think it won't hurt giving some information.

The report was about the emblem disappearing when a new zone was loaded (i.e. entering/leaving towns and dungeons, using portals and channelers). The problem was that the player actually wasn't allowed to wear the emblem yet.

You can start using your guild's emblem when you are

a) level 20 or
b) a crafter with skill 400.

Note: It doesn't help asking a fellow guild member to put the emblem on your items when you can't do it on your own because then the situation described above will happen.