What "stopped out" means
Being stopped out means price reached your stop-loss level and the trade was closed to limit your loss. That is the stop working as intended — it is not a malfunction, even though the result is a loss.
Note this is different from a 'stop-out' caused by margin, where the broker force-closes trades because your account ran too low. This page is about your own stop-loss firing.
Why it happened
Price simply moved against you and hit the level you chose. Sometimes it was a normal fluctuation, and the stop was placed too tight for the instrument's usual movement (its volatility). Sometimes a sharp move or a news spike ran through it. And remember the relevant side (bid for buys, ask for sells) plus the spread can touch a stop before the mid-price looks like it did.
How to place stops that survive noise
Base the stop on the market's structure and typical range, not on a round number or on how much you are willing to lose alone. Give it room beyond normal noise, and then size the position so that this wider stop still keeps your risk per trade small.
Avoid placing stops exactly at obvious levels many others use, where price often briefly overshoots.
How to prevent needless stop-outs
Match stop distance to volatility, allow for the spread, and avoid trading tiny stops through news. If you are being stopped out constantly, the problem is usually stop placement or entry timing, not bad luck.
Broker's fault or yours?
Almost always yours in the sense that the stop did what you asked. Genuine concern only arises if fills sit far outside real market prices repeatedly — otherwise, treat a stop-out as feedback on your placement.