Key blocking
Also called key jamming · anti-ghosting
Key blocking is when a keyboard ignores a key you pressed because registering it could produce a ghost key. The key works on its own but drops out when held with certain other keys.
Hold the combination you use in a game on the keyboard test. If one key never lights up while the others are held it is being blocked, not broken.
In practice
Key blocking is a keyboard protecting itself from ghosting by refusing a key it can’t be sure about. The key works alone and fails only alongside certain others. In games that looks like a jump that won’t fire while running left or a reload that ignores you while strafing. The combination that fails depends on how that keyboard’s grid is wired, so it differs between models.
How to check
In the keyboard test, hold the keys you were using when the problem happened and then press the one that failed. If it never lights while the others are held but works alone, it is blocked. Try nearby combinations too. Remapping the game action to a key on a different part of the grid often avoids the clash entirely.