Your first stun lasts as expected, but the next one ends quickly. It may not be lag or a failed input. Some games reduce repeated crowd-control effects on the same target through a diminishing-returns system.
The important details are the effect’s category, the target and the rules currently in use. Two abilities with different names may share a category. Conversely, an effect that looks similar on screen may be handled differently by the game.
Use the game’s current rules
Blizzard’s Midnight pre-expansion update notes describe changes to World of Warcraft’s diminishing returns, including differences between player-versus-player and player-versus-environment behaviour. This is a dated example of the mechanism, not a universal table for every game or a promise that later patches leave it unchanged.
Before copying a control sequence, check which version and mode the guide describes. A strategy demonstrated against a player may not apply to a boss, and ordinary enemy behaviour may not describe a special encounter.
Review the sequence, not just the final stun
In a replay or combat log, identify what affected the target immediately beforehand. A teammate’s ability may have started the relevant sequence before your own action. Record the category and timing where the game exposes them, rather than assuming your button was the first control effect.
During practice, coordinate one effect at a time and observe the result under the current rules. Keep the target type and game mode consistent. This helps distinguish diminishing returns from an immunity, a dispel or an interrupted cast.
The team question becomes “Which control effect is useful now?” rather than “Who can press another stun?” Better coordination can preserve an important ability for a later moment. It also avoids blaming a teammate for a short duration that the game’s system had already determined.