Guild Wars 2

situation:
I was at Pact Rally Point in Straits of Devastation, there were 5 or 6 other players there, 2 were doing the skill challenge (Vigil Tactician).
I was standing near the scout (explorer Vorb) with a view on the skillpoint challenge.

The game did not show the two players doing that skill point even though it was barely 20 or 30 meters away from me and there were only a few npcs and other players around.

When i got closer to the skill challenge the game did show the two players, and still showed them as i moved back to my previous location while keeping the fight in my view.
But once i turned around and then looked back again at the skill challenge, the game did again not show the two players doing that skill challenge.
The game does not always do this that badly, it seems kind of random.

I have LOD distance at “high”, all other settings at medium or low. Graphics card is a AMD Radeon HD6800 series 1GB.