Since the patch update on 3/16 guild chat is not working consistently for a number of guilds. The guild chat seems to stop working when players initially login, or change characters, or change chat tabs.
These are temporary workarounds our guild found:
1) Change your status to invisible then online. Y key, (friends list) click green square, and then invisible.
2) Stop representing your guild then click represent again.
3) If the above doesn’t work try restarting your game.