Your order might be getting rejected due to multiple reasons such as insufficient funds, stock group change, stock not available for trading, connectivity issues, etc.
To know the rejection reason, Go to the order book
To go to your order book please click on View Order/Trade Reports and select Order Book or simply hit F3.
Below is an example of the order book window.
If your order has been rejected, you can view the same in the "Rejection Reason" column.
