I am at shards landing I pulled 3 mobs with a merc melee dps up set on burn. I killed all 3 of the mobs and my merc is just sitting there doing nothing.
Yea I have noticed it a few times since the new expansion and the new merc when in older zones. Not sure I have ever seen it happen in new zones.
Some folk say it has to do with hilly terrain but I have experienced this on flat ground as well.. Assuming the group rolls are set correctly.
Make sure they are not leved. If mercs are at enough of an elevation difference they will either not follow you or not assist/heal. If you have leved your group for some reason, and no longer need it, click the lev on the mercs off
This seems to be happening for me for DPS merc in Plane of Sky sometimes, it's unusual. If the DPS merc is behind the mob, he attacks fine. If the mob faces the merc, it sporadically stops attacking.
Melee merc has been doing this to me for a while. Sometimes taking levitation off helps. If that doesn't, setting to Passive and then back to Balance or Burn gets them interested in fighting again. Usually I just have to toggle on/off once and they behave. Occasionally they don't and just stand there doing nothing unless I dismiss and then revive after the timer is up.