MetaTrader charts are established using the Bid Price. The closing of Buy Orders is triggered by the Bid Price, hence you will see the chart hitting the Stop Loss (SL).

On the other hand, Sell Orders are triggered by the Ask Price which is not visible until you enable it. That's why Stop Losses of Sell Orders may be triggered on prices not visible on the chart. 

The Ask Line can be enabled by right-clicking on the chart, clicking Properties, choose a colour to represent the Ask Line and click OK to save the settings.