Limited volume
Your order won’t be filled if there aren’t enough shares available at the specified price or number. This occurs most frequently with large orders placed on low-volume securities. Keep in mind that there must be a buyer and seller on both sides of the trade for an order to execute.
Market open conditions
If a market center starts trading later than market open, you may see delays in your order getting filled. Also, if trading volatility is high, it might prevent the order from filling immediately once the market opens.
When it comes to options, listed equity options don’t begin trading until trading has begun at the primary listing exchange for the underlying stock. This is most common for NYSE symbols, where the opening auction may not occur until a few minutes after 9:30 AM ET.
Extended hours
During extended-hours trading, orders may not fill or have limited tradability due to lower volume and wider price spreads as compared to trading during regular market hours (9:30 AM–4 PM ET). The list of symbols that are eligible for fractional orders may differ from the list of symbols that are eligible for whole-share orders.
Overnight hours
During overnight hours trading, only whole-share limit orders can be traded, and some orders may not fill or have limited tradability due to lower volume and wider spreads as compared to trading during regular market hours. The list of symbols that are eligible for trading during overnight hours will be a subset of those eligible for trading during extended and regular market hours.
Limit or stop price hasn’t been reached
If your stop or limit price hasn’t been reached, your order will remain pending until there’s a buyer or seller willing to trade at your specified price. Keep in mind, the price displayed on the Tradesk app is the last trade price on a Nasdaq exchange (the Nasdaq Stock Market, NASDAQ OMX BX, or NASDAQ OMX PHLX) or the last trade price on BOATS ATS, not the price at which shares are currently available. This means that if there are no shares currently available at your limit price, your trade may not execute—even if your limit price is the same as the price displayed.
Limit price reached, but order not filled
The success of your limit order isn’t necessarily due to time and price priority on the markets. The order fill rate depends on a number of elements, like market volatility, size and type of order, market conditions, and system performance.
Unstable market conditions
When there is a massive price drop or spike and no purchases or sales, respectively, a market order may not be filled. While rare, this can occur when there are market halts for price volatility.