It doesn't mean if you are an investor you will always get a good profit, sometimes the market is not on their side that's why they always have a hard time to manage their profit over losses. Enough knowledge about the market is a very important aspect here or even more on that so that you will not having a hard time to analyze everything.
That is a problem that I have with many traders, they seem to think that to be a good trader you need to win all the time and that is not true, as long as you can get consistent profits then you are a good trader and if that means accepting a few losses then that is something you need to be willing to accept otherwise despite your ability as a trader sooner or later you are going to bust, because at some point you are going to take a bad decision just because you lost a few trades in a row.