There are several reasons this may happen. If you are a buyer, make sure that you have enough cash in your account to fund your BID. If you are a seller, make sure you have enough shares available to cover your ASK. If your BID was below the market-clearing price, or your ASK above it, then your transaction will not be completed. Also if you submitted an equal BID or ASK as someone else, orders will be filled based on availability of shares in the market.
Our partner broker-dealers will try to make the market as active as possible. The more supply and demand there is, the more likely you can buy or sell when you want.