mirkwood reflecting pool

hello. i want to repeat the "hilltop assault" quest from the mirkwood landing, so i traveled to ost galadh to use the reflecting pool there.
when trying to travel to hilltop assault, it says "using it has no effect". why?
note that i didn't done any other intance in mirkwood, so i can't compare hilltop assault to other intances, and i don't have the mirkwood quest pack/expansion.