When a market order is not filled, this is usually because the market is less liquid or there is a lesser volume of assets than normal. For a market order to be filled, there needs to be a willing buyer and seller on both ends.
For example, if you place a large market order for a particularly low-volume share, such as a penny stock, there may not be enough shares available at the current price. This means that your order won’t be filled, and you will have to wait until there is more liquidity within the share market to re-submit your order.
A market order may also not be filled due to volatile or unstable conditions of the market. For example, if there is a large jump or drop in an asset’s price, sometimes the market will halt. When this period of volatility is over and market conditions are more stable, you may then place a market order once again, as the previous would not have been filled.