Orders can be rejected for a number of reasons.

Value of order too low

Your order will be rejected if the value of the order you placed wasn’t high enough to cover the cost to buy the shares. Although we give an estimate of the price when you place an order, you may find the actual price you need to pay may be different due to the price fluctuating between the time you placed the order and when we execute it. Your order may be rejected as you can’t buy less than one share (although you will be able to soon, when we roll out fractional shares!) 

No valid quotes

If you experienced a rejection and your order was of a high enough value to cover the trade, it could be because we were unable to retrieve a valid quote from the market makers, i.e. no one wanted to buy or sell your shares on that occasion or were unwilling to offer a good price. When this happens, your best bet is to leave it a short while and try again later. 

We have a handy blog post which explains this in more detail here

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