It can happen that a countermeasure does not trigger even though all requirements are met. This can be the result of regular gameplay and can be in order with the game rules.
Enter the C6N SAIUN!
The special ability of the Japanese C6N SAIUN is to prevent any countermeasures to be triggered.
If a C6N Saiun is on the battlefield, then no countermeasure will trigger - neither yours nor from your opponent. If your countermeasure didn't trigger, then review the battlefield to see if there is a Saiun on the board.