Line of sight
From Guild Wars Wiki
Line of sight refers to your ability to see an object with no physical obstructions. Projectile spells and ranged weapon attacks require a clear line of sight to the target to be successful. Intuition is currently the only method of ensuring you have a clear line of sight besides firing a test shot.Occasionally, due to pathing inconsistencies as well as artificial and/or invisible obstructions, a player will have a clear line of sight and remain unable to strike his/her target. There is little that can be done to correct this, other than moving and finding a new firing position.

