There might be several reasons, depending on the order type:

For Limit Order, the order will not be executed unless the order price reaches the current price.For Stop-Market Order, the order will not be executed unless the current market price reaches the Trigger Price.For Stop-Limit Order, the order will not be executed unless the current market price reaches the Trigger Price and the Limit Price. The margin between the Trigger Price and the Limit Price of the order will increase the probability of your order to be matched and executed. For these reasons, there is no guarantee of execution of the order in Limit and Stop Orders.

You can check your unfilled orders from the "My Open Orders" panel. If you have an open order that you want to cancel, you need to click on the "X" on the far right of the line with the order.