There are a number of reasons why an order in your trading account might get cancelled unexpectedly. You can learn more about each order type and the common causes for cancelled orders below.

Market orders

There was a trading halt placed on the security you were trying to trade.
A trading halt happens when an exchange stops the trading of a security for a particular amount of time. During a trading halt, orders placed for the halted security will be cancelled.

A recent deposit got rejected by your bank.
If your deposit was rejected by your bank, this means that your bank will pull back funds from the rejected deposit. While this is in progress, you are unable to place an order, and any pending orders will automatically be cancelled.

You have insufficient funds in the account to cover your trade.
If you don't have enough funds to cover the full balance of your trade in the event of a sudden price movement, your order will be cancelled.

The market order for an option has low liquidity.

Market orders for options with low liquidity may be unfilled or partially unfilled and will be cancelled.

Limit orders

Your order did not fill because the limit price was not met.
For a limit order to fill, the price of the security must meet or be better than your set limit price. Remember that if you're selling a security, the limit price must be higher than the market price. Similarly, if you're buying a security, the limit price must be lower than the market price. If the limit price is not met, your order will be cancelled.

Your order did not fill because there was not enough volume at your limit price.
When you want to buy a stock, someone in the market must be selling the stock at a price you are willing to pay. If there is not enough volume for your stock at your limit price, then your order will not be able to fill and will be cancelled.

There was a trading halt placed on the security you were trying to trade.
A trading halt happens when an exchange stops the trading of a security for a particular amount of time. During a trading halt, orders placed for the halted security will be cancelled.

Your order was cancelled because the limit price was too far from the quoted price.
If the limit price is very far off from the quoted price, your order may be cancelled.

Your order did not fill because the limit price was not met for an odd lot.
The quotes for most securities are made based on a standardized number of units (usually 100, referred to as board lots). If you submit an order for an odd lot of shares with a limit price, your order will only be filled if the limit price meets (or exceeds) the National Best Bid or Offer (NBBO). Please note, that the NBBO may not match a specific quote provider’s data, as the NBBO takes into account all protected markets the symbol trades on, whereas a quote provider may only consider the data on one exchange.

The TSX and TSX-V consider the below order sizes to be board lots, anything that is outside of this would be considered an odd lot.

US markets (NASDAQ, NYSE etc.) consider 100 shares to be a board lot regardless of the price.

Your order was cancelled because the security underwent a corporate action or dividend distribution.

If a security goes through a corporate action or dividend distribution, the limit order you have submitted prior to the corporate action's ex-dividend date may either be rejected or adjusted to compensate for the change in price to the security. For example, in the case of a dividend distribution, if company XYZ pays a dividend of $0.10, open limit orders set for $10 prior to the ex-dividend date can be adjusted to $9.90 to reflect this change. This is because dividends are paid out of the cash that the company has available, and the company is subsequently no longer holding that cash. Therefore, the implied value of the company should also drop by the value of the cash dividend that was paid out and is reflected in the trading price.

A recent deposit got rejected by your bank.
If your deposit was rejected by your bank, this means that your bank will pull back funds from the rejected deposit. While this is in progress, you are unable to place an order, and any pending orders will automatically be cancelled.

Stop-limit orders

Your order did not fill because your stop price was not met.
Your stop price must be hit before your limit price for your stop-limit order to fill.

Your order did not fill because the limit price was not met.
For a stop-limit order to fill, the price of the security must also meet or be better than your set limit price. If the limit price is not met, your order will be cancelled.

Your order did not fill because there was not enough volume at your limit price.
When you want to buy a stock, someone in the market must be selling the stock at a price you are willing to pay. If there is not enough volume for your stock at your limit price, then your order will not be able to fill and will be cancelled.

 There was a trading halt placed on the security you were trying to trade.
A trading halt happens when an exchange stops the trading of a security for a particular amount of time. During a trading halt, orders placed for the halted security will be cancelled.

A recent deposit got rejected by your bank.
If your deposit was rejected by your bank, this means that your bank will pull back funds from the rejected deposit. While this is in progress, you are unable to place an order, and any pending orders will automatically be cancelled.

Fractional orders

There was a trading halt placed on the security you were trying to trade.
A trading halt happens when an exchange stops the trading of a security for a particular amount of time. During a trading halt, orders placed for the halted security will be cancelled.

A recent deposit got rejected by your bank.
If your deposit was rejected by your bank, this means that your bank will pull back funds from the rejected deposit. While this is in progress, you are unable to place an order, and any pending orders will automatically be cancelled.