A stop-loss is meant to be the moment a losing trade stops being your problem. Most of the time it closes at, or very near, the price you set. The times it doesn't — filling a few pips or a few dollars worse than expected — are where "why didn't my stop fill at my price" comes from, and the mechanics behind it are predictable, not a malfunction.
A stop triggers an order, not a price
A stop-loss isn't a promise that you'll be closed out at your exact level. Once the price touches your stop, it becomes a market order that fills at the next available price. In a normally liquid moment that gap is usually a fraction of a pip and goes unnoticed. In a fast or thin one, it can be materially worse — that gap between your stop level and your actual fill is what's called slippage. See order types explained for how a stop order differs from a limit or market order more generally.
What actually causes the gap
- Thin liquidity. Outside an instrument's most active hours, or in a less-traded pair, there may not be a buyer or seller waiting exactly at your stop level. The order fills at the next price where one exists.
- Fast-moving prices. During a scheduled data release or a sudden headline, price can move through several levels in the time it takes an order to route — the fill lands wherever the market actually is by the time the order executes, not where it was when the stop was set.
- Clustered stop levels. Traders tend to place stops around the same round-number levels. When price reaches one, a wave of stop orders can execute together, pushing price further before it settles — a late order in that wave gets a worse fill than an early one.
Slippage runs both ways
The complaint is always about a stop filling worse, but the same mechanism can fill a take-profit or a fresh entry better than expected on the other side of a gap — that direction just gets less attention because nobody writes in about an unexpectedly good fill. What's worth checking is whether your broker's order-execution policy treats both directions the same way. Regulators have taken action in the past against firms whose execution practices applied slippage asymmetrically — passing on worse fills to clients while keeping better ones for the firm — so this isn't a theoretical concern. Your broker's execution policy document, not its marketing page, is where that's actually spelled out; verify what it says on the official regulator register for your broker's licence rather than assuming symmetry.
Weekend and calendar gaps are the extreme case
Markets that close — most forex pairs over the weekend, for instance — can reopen at a materially different price from where they closed. A stop-loss can't protect against that kind of gap, because there was no trading, and so no possible fill, at any price in between the old level and the new one. It isn't a broker failing to honor your order; there was simply no market to fill it in.
Guaranteed stops trade certainty for cost
Some brokers offer a guaranteed stop-loss that fills at exactly your specified level even through a gap, usually for a fee or a wider spread on that order. It's a reasonable tool for a position you're holding through a known high-volatility event, but it isn't the default order type, and it costs something specifically because it removes the risk described above. See order types explained for how it compares with a standard stop.
A worked example
Say EUR/USD is trading at 1.0850 and you set a stop-loss at 1.0830 on a 1-lot position. A surprise rate decision hits and price drops sharply, gapping from around 1.0845 straight through your stop level to 1.0805 within seconds. Your order becomes a market order the instant price crosses 1.0830, but the next available price by the time it fills is 1.0818 — 12 pips worse than planned, roughly $120 on a standard lot. Nothing malfunctioned; the market simply moved faster than any order in the queue.
What you can actually control
- Be aware of scheduled high-impact data for the instrument you're holding, and size positions with the possibility of a worse-than-planned fill already built in.
- Use risk management basics — sizing to a stop that assumes some slippage is safer than sizing to the exact level and being surprised.
- Consider a guaranteed stop for a position you're holding through an event you know is coming, if the cost is worth the certainty to you.
- Read your broker's execution policy, not just its spread advertising.
Educational content only, not financial advice. Illustrative figures only — check live pricing and your broker's execution policy for the real numbers.