Direct answer
MT5 Orders can behave differently when market conditions change the path from “order placed” to “order executed.” In practice, the differences show up most clearly with changes in liquidity, volatility, spread/costs, and whether your order can be filled at (or near) the requested conditions. Because different brokers can apply different execution policies and because markets vary over time, you should treat the observed behaviour as conditional, not universal.
Mechanism and definition
An “order” in MT5 is an instruction that relies on a later matching of your order requirements (price, side, and triggering rules) with available market prices and the execution environment. The behaviour you notice—such as whether an order fills fully, partially, or not as expected—depends on two broad parts:
- Stable logic (your order intent): what the order type requires (for example, whether it triggers only after a specific price level is reached, or whether it attempts to execute immediately at the available market price).
- Variable market conditions (how prices change): the order’s target may become unreachable, move past the trigger quickly, or face limited liquidity.
A key concept is that many outcomes depend on the gap between the market state when you place the order and the market state at the moment execution happens. Even without any “strategy” changes, this gap can widen or narrow.
Evidence through a comparison of typical conditions
Below are common market-condition scenarios where different behaviour is more likely. These are explanatory mechanisms, not predictions.
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Low liquidity / thin order books: When there are few willing buyers or sellers near your desired price, the market may jump to the next available price level. That can cause fills at prices that differ materially from what you expected at placement.
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High volatility: Large, fast price moves can skip over trigger levels or move through them before execution completes. That increases the chance of orders filling at worse prices, not filling, or filling partially.
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Wider spreads and higher effective costs: If the spread widens, the executable price available to your order can be further from the mid-price you might mentally use for reference. This can make “the same order rules” behave differently as costs widen.
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Price gaps around news-like events (rapid repricing): When prices reprice abruptly, the market may move beyond your order’s intended constraints between the time you submit and the time the execution engine processes the order.
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Limited ability to meet constraints (effective rejection): If your order type or constraints require a specific price/trigger relationship that no longer exists at execution time, the order may not execute as intended. This is a limitation of the underlying matching process rather than a “model error.”
Across these scenarios, the shared pattern is that the market can change faster or become less tradable than your order’s assumptions about immediate availability.
Limitations and risks (failure modes)
Material limitations you should account for:
- Slippage: The difference between the price you expected (at submission) and the price you actually receive (at execution).
- Partial fills: If liquidity is insufficient, only part of the requested quantity may execute.
- Order not filled despite being valid at placement: Execution timing and market movement can make the required condition false by the time the platform/broker can match it.
- Execution-policy differences: Even with identical market conditions, a broker’s execution setup (for example, how it handles market orders, re-quotes, or time-in-force) can change the observable outcome.
Because you may not know the broker-side execution rules in full, you cannot conclude that “MT5 behaves one way” without checking what actually happened in your own environment.
Verification and next question
To verify conditional behaviour independently, compare your specific order events (placed time, executed time, fill price(s), and whether it filled fully/partially) across multiple test periods that differ meaningfully in liquidity and volatility. Focus on measurable fields in your execution history rather than on what you assumed the market “should” do.
A useful next question is: Which MT5 order type are you referring to (market, limit, stop/trigger-based), and what exact differences did you observe (not filled, partial fill, different fill price, different trigger timing)? That details the correct conditional explanation without guessing outcomes.