The target is close enough according to the distance indicator, yet the ability refuses to activate. Moving one step around a pillar suddenly makes it work. Range and line of sight are different checks: a short distance does not necessarily provide an unobstructed path.
Blizzard’s Dragonflight Season 1 arena description gives a concrete historical example, describing ramps, pillars and a wall used to break opponents’ line of sight. The exact interaction of an ability with terrain still depends on the game’s rules and version.
Read the failure before repeating the input
Check whether the message says out of range, no line of sight, invalid target or something else. Repeatedly pressing the same button can hide the distinction and leave you unaware of the actual condition preventing the action.
In a safe practice area, compare two positions at similar distance from a target: one with a clear path and one behind an obstacle. Observe the ability’s feedback. This is a test of that ability in that setting, not proof that every spell, projectile or area effect follows the same rule.
Terrain affects allies too
A pillar that blocks an opponent may also obstruct an ally trying to support you. Before moving behind cover, consider the team’s positions and communicate if the game mode makes that useful. Defensive terrain is not automatically beneficial from every angle.
Camera visibility can also mislead. Seeing a target on screen does not always mean the game’s collision or targeting check considers the path clear. Conversely, some abilities have documented behaviour that differs from an ordinary direct attack.
When reviewing a failed attempt, record the ability, obstacle and error message. Check the official ability description or current support information before concluding that a familiar-looking wall is bugged.
The useful habit is to diagnose position in two dimensions: how far away the target is, and what lies between the relevant points. Once those questions are separate, a small repositioning decision becomes more deliberate than simply moving closer and hoping the button starts working.