Why did my stop-loss order not trigger?

A stop-loss only activates when the exchange’s traded price actually reaches your trigger level, so the most common reason it didn’t trigger is that the stock simply never touched that price during the order’s validity, even if it looked close on a delayed chart. A gap can also cause this in the opposite direction: if the stock gaps straight past your trigger level between one tick and the next (common after news or at market open), some order types can trigger and execute simultaneously rather than in two separate steps, which can look like it didn’t trigger in the expected sequence. Low liquidity in the specific stock or option contract, an order placed outside market hours without after-market support, or an expired order validity are other possibilities. You check the actual traded price history around the trigger level and the order’s status log rather than relying on a chart’s visual appearance, since charts can be delayed or use a different price feed than the exchange’s actual tick data.