The reason I ask the above question is because I am working on a strategy that works on daily candles and places a limit order to buy and to sell. In back testing the buy orders are all working fine and 99% of the sell orders are working fine but just occasionally it seems that a sell order is missed.
Here is an example:

The little blue arrow is the buy which is 5 pips below the sell limit order price which is supposed to be at the close price of the candle before – but it does not sell there and stays on the market all day and sells at the close of the candle/open the next day which is the second exit for the strategy if the target has not been hit during the day. It is as if the SELL LIMIT order is not placed in the back test. In this example is was actually more profitable not to sell but that is not the point!
Here is an example of it working as it should in the same back test:
