In the part of the Rivenspire main quest-line called "The Blood-Spattered Shield" there is a part where you have to go to a tower. Once you go there, you're given the objective to "defend the tower." I tried to do this quest today, and at the "defend the tower" part, it never triggered. You are supposed to get mobs rushing toward the tower, which you have to defend against, only it wouldn't trigger and the mobs never appeared. I stayed for a long time, moving around to try to get it to trigger, and it never did. I left Rivenspire completely and came back, and it still wouldn't trigger. I closed out the game and came back, and it still wouldn't trigger.

I did this quest about four months ago with a different character, and it worked fine, so not sure why it stopped working. You might want to check this out.