In certain situations, where there is extremely bad news regarding the company, the stock prices drop rapidly. Due to this, the stock hits lower circuit limits continuously on a daily basis and no new people come in to buy these shares. This generally happens in penny stocks which have no liquidity.

Similarly, when there is a lot of buying pressure & demand for a certain stock, there may be a lot of bids but no one willing to sell these shares.

The market depth will look as depicted in the below images -

This is why even though the order is placed successfully and is an open order it may not get executed.

When you place an order during normal market hours , order matching on the exchange happens on a price-time priority . This means that orders get executed on a 'first come first serve' basis (queue system). If there are people who have placed orders before you, your order will be executed only if the orders placed earlier gets filled.

To be ahead of the queue, what you can do is -

1. Place an AMO - Limit/Market order after 3:45 PM. (Market orders have a higher chance of being filled)
2. Place a market or limit order at the pre-market i.e. at 9:00 AM

Placing a pre-market order has a better chance of being executed than an AMO. But placing an AMO is more convenient as you can place it anytime between 3:45 PM to 8:57 AM for NSE & 3:45 PM to 8:59 AM for BSE and not wait for the pre-market timings.

In spite of placing an AMO or pre-market order, there is no guarantee that your order will get executed. This is the same across all brokers.