Your Pending Order may not be executed for a variety of reasons:
Price Has Not Been Reached
The instrument hasn't reached the limit price yet, so your pending order has not been executed.
Price Was Reached and Order Not Filled
The success of your limit order isn’t necessarily due to time and price priority on the markets. The order fill rate depends on a number of elements, such as market volatility, size and type of order, market conditions, and system performance.
Unstable Market Conditions
When there are drastic price fluctuations, spikes or drops, and no sales or purchases, your pending order may not be executed.
While these scenarios are rare when they occur the market will be halted for price volatility and your order will remain pending.
Extended Trading Hours
During extended trading hours (9–9:30 AM and 4–6 PM EST), orders may not fill due to lower volume and wider price spreads when compared to normal trading hours. Some instruments may also have limited trading ability during extended trading hours.