An order, though placed successfully, may not be executed because of the following reasons:
There is no matching bid / ask on the order book at or better than the limit price entered.
Your order is far behind in the queue despite having a matching bid / ask in the order book. Orders get filled on a first come first serve basis, and the matching bid / ask might have been exhausted by the orders which were ahead in the queue.
There is no matching bid / ask on the order book due to stocks hitting lower / upper price circuits.