There may be instances when your market order does not get filled. This can be attributed to 2 main reasons: 

Trading Halt 

A trading halt occurs when there is a temporary suspension of trading of a particular security in the stock market. When a trading halt happens, your market order may appear to be stuck at "pending" status and does not get filled as expected. To see if a security is under a trading halt at any point in time, you may check the security information on the exchange market's official website(s).

Lack of Volume 

A market order may not be filled when the security is less liquid. For example, if you place a large market order for a particularly low-volume security, there may not be sufficient shares available at the current price to fill your market order. This would mean that your order might not get filled and you would have to wait until there is more liquidity in the market. Your order would remain in "pending" status until it expires or is filled later on.