They take too many positions with too little information. They take small profits, not riding gains as they should, and tend to stay with losing positions. Too many traders do not apply money management techniques. They have no discipline, no plan. Many also overstay when the market goes against them, and won't limit their losses.
Many traders are undercapitalized.
They trade positions too large, relative to their available capital. They are not flexible enough to change their minds or opinions when the trend is clearly against their positions. They don't have a good battle plan and the courage to stick to it. Don't make trading decisions based on inside information. It's illegal, and besides, it's usually wrong.