Experienced traders also have the chance to have losing trades, despite the fact that they have enough knowledge to understand what they are doing, but there are manipulations in the market and it is not even a matter of luck, although in the case of a beginner, luck may matter more than in the case of an experienced trader.
There is no trader who can have perfect success rate in trading and this raises importance of manage their trading capital, and if they succeed, get profit through trading, it's time for them to withdraw their profit and secure their original capital. If they can keep their original safely by withdrawing part of their profit, they will not lose initial capital even they lose all in one bad, unlucky trade.
The most important thing is that the trader is successful enough in the long term, so that most of the trades are successful and a few losing trades do not greatly affect their results.
To succeed in long term trading, managing their trading capital is most important. If they always use all capital for trading, they can win big or lose big and there is possibility to lose all their capital one day.