An order can be cancelled for various reasons.

• For example, if you entered a day order after the closing of the stock exchange, at the weekend or on a public holiday.
• Non-executed orders automatically expire after their validity date. 
• The limit price might have been incorrect or deviated too much. Most exchanges do not allow a limit price that deviates more than 10% from the last price (or from the Bid or Ask).
• The London Stock Exchange quotes in pence, so your limit price must also be entered in pence (GBp), not pound (GBP). Do not put dots between thousands and use a comma for decimals. One pound = 100 pence.
• Some exchanges automatically cancel outstanding orders on their ex-coupon dates because the price of the security then decreases by the coupon value and outstanding limit prices do not take this into account.