Most of the time a trade that doesn't go through has been stopped for one of three reasons. Check which one you saw.
The price moved
Your order takes a moment to travel from your device to our server. If the price moves a little in that time, the trade fills at the new price. This is called slippage, and small amounts of it are normal, especially in busy markets. If the price has moved a long way, the trade is rejected instead and a message tells you so, so that you're never filled far from the price you saw.
Slippage happens most in volatile markets and on slow connections. If it happens often, try a more stable connection.
An error code appeared
The code tells you what stopped the trade: for example the market is closed, the amount is outside your limits, or liquidity is low. See Error codes and messages.
Nothing seemed to happen
Check the Open Trades tab under the chart before trying again. If the trade is there, it went through and you just didn't see the confirmation. If it isn't, refresh the page and try once more.
If a trade still won't go through, contact support with the asset, the expiry, the time and any code you saw.
Related: Error codes and messages · The trade buttons are greyed out