Your order may not be executed even after being placed successfully due to certain market-related factors. The most common reasons are explained below.

1. Price–Time Priority System

As exchange follows a Price–Time Priority (FIFO) system for order matching. This ensures fairness and efficiency during pre-open and regular trading sessions.

How Price–Time Priority Works

Price Comes First:

• For buyers, the highest bid price gets priority.
• For sellers, the lowest ask price gets priority.

Time Breaks the Tie:

• If multiple orders are placed at the same price, the order that was placed earlier is executed first (First-In, First-Out).

 Example Scenario 

Underlying Asset: Reliance Futures

Exchange: NSE

Matching Rule: Price–Time Priority (FIFO) 

Buy Orders:

• Buy Order 1: 100 lots @ ₹2800 (Time: 9:40 AM)
• Buy Order 2: 50 lots @ ₹2800 (Time: 9:45 AM)
• Buy Order 3: 200 lots @ ₹2799 (Time: 9:42 AM)

Incoming Sell Order:

• 150 lots @ ₹2800

Matching Process:

• The system checks the best price, which is ₹2800.
• At ₹2800, Buy Order 1 and Buy Order 2 are eligible.
• Buy Order 1 gets priority because it was placed earlier. 

Execution Outcome:

• 100 lots are matched with Buy Order 1 (fully filled).
• Remaining 50 lots are matched with Buy Order 2 (fully filled).
• Buy Order 3 remains unexecuted as it is at a lower price (₹2799).

Key Takeaway:

Orders are executed first based on the best price, and if prices are the same, based on the earliest time of placement.

Order Matching During Different Trading Sessions

Pre-Open Session for F&O (9:00 AM – 9:08 AM)

• Orders (buy, sell, modify, cancel) are collected.
• Matching happens using price–time priority to determine the opening price.

Regular Market Session (9:15 AM – 3:30 PM)

• Orders are matched continuously in real time.
• Best price gets priority, followed by earliest order at that price.

Important: Even AMO or pre-market orders do not guarantee execution.

2. Low Liquidity

If an F&O contract has low trading volume, there may not be enough buyers or sellers available to match your order, leading to non-execution.

3. Limit Price Not Reached

• Limit orders execute only when the market price reaches or improves upon your specified price.
• If the market does not touch your limit price, the order will remain unexecuted.

4. Circuit Limits and Volatility Controls

• Unlike equity stock, F&O contracts do not have circuit limits. But  they may still face restrictions or special measures during periods of high volatility.

5. Insufficient Margin

• F&O trading requires sufficient margin in your account.
• If adequate funds are not available, the order may be rejected.

Example: Buying options requires payment of the option premium and any additional margin, especially before physical settlement.

6. Trade Restrictions or Surveillance

• Exchanges may place certain F&O contracts under trade restrictions or surveillance.
• This can limit or temporarily restrict trading in those contracts.