Attacking with Heartseeker sometimes won’t work (at all). I’m not exactly sure what’s causing it, but I know it’s terrain related. It’s either due to the mobs being behind terrain, or when the player is behind terrain. Whatever is causing it, it will 100% stop Heartseeker from firing. I’m using standard mouse and keyboard (not WASD movement).
I’m trying to figure out exactly what’s causing it, so I’ll update this post if I figure it any specifics.
[edit] Thus far, I’ve only been able to replicate this bug when the player, the mobs, and the cursor are behind terrain that won’t disappear (like some pillars). In the area “The Tundra” in the Divine Era, just north of the waypoint, there’s a circular area before the bridge to The Temple of Heorot, and along the southeast and southwest edge of that circular area, are pillars that don’t disappear when the player sits behind them. When the player, the mob, and the cursor are behind the pillars, Heartseeker won’t fire. If you click on an open area (not under the pillar), Heartseeker will fire, even if the player and mob are behind the pillar.
[2nd edit] Heartseeker also visually disappears when shot at destructible terrain features (like barrels and piles of wood planks) which drop loot. Heartseeker still functions and destroys those things, it just disappears visually.