Do you use the stop loss timeout?
When the stop loss timeout is active, the price should remain below your stop level for X minutes. Consider the delay carefully as there's no universal value for all situations, even on the same pair. If you set a long delay, the trade might close much lower.
Don't use the stop loss timeout with limit stop loss. After the time out, the price can be much lower than your limit order price, and your stop loss will often end skipped.
Do you use Trailing Stop Loss?
That will be the reason if your trade closes earlier than you expect. The trailing feature moves the stop level after the price peaks, so the stop loss price eventually changes more and more.
This article explains how the Trailing Stop Loss works, please check it and reconsider if you want to use TSL for further trades.
Did the price take a sharp rise or drop?
When the price drops or rises very fast, a market stop loss might execute at worse prices, and the limit stop loss might not execute at all.
Check the next section to find out more about limit stop losses.
Market orders are there to buy or sell something as fast as possible at the best available price right now. When the price changes very fast, the term "the best available price" can be equal to "the worst price."
Do you use a limit order for stop loss?
The limit stop loss order consists of two parts:
The stop price is the price to determine when to place the limit order.
The order price is the price of the limit order that goes to the order book when the coin price reaches your stop price.
The limit order allows you to control the worst price you're willing to sell or buy your coins, but it also should be filled. The order is likely to be stuck on the exchange's order book if the price doesn't reach it or moves fast through your stop level. To prevent the limit order from being stuck:
Set the order price a bit higher than the stop price for short positions (Smart Cover).
Set the order price a bit lower than the stop price for long positions (Smart Trade, Smart Sell).
Don't set the order price higher than stop price for long positions or lower than stop for short trades. When you do this, the price should take additional bounce to reach and fill your limit order, which means it'll often be skipped.
Also, if you set the order price equal to the stop price, the price might skip your order on fast moves.