You have contradicted yourself in your own post.A small capital is like demo trading , like going to a casino without the emotions and phsychology , you won't experience real phsychology until you trade large accounts.The £300 loss means nothing , compared to net worth.
Trading is hard and a competitive mental sport , that is why they failed.
You say they failed because its a hard mental game and then say none of the traders are experiencing emotional or psychological (mental) stress because of the small size of the account.
So 100% failed on purely technical reasons, which is a lot more than 20% random number you imagined.
Also, if your going to pretend you know something about trading 'psychology' at least learn to spell the word correctly.
Sometimes I wish 'cut and paste' hadn't been invented.