Changing the plan or trying to increase your position in the middle of the trade is one thing that can bring about unfavourable result especially when its basically to increase your profits because the trade is playing out as predicted. this can also be related to greed . the best thing is to follow your plan and be satisfied with whatever profits you make. you cant make all the profit at once but you can lose all your capital in a single trade because of greed or over ambition. if your strategy has DCA then you can use it or else just accept your loss when your trade hits the stop loss and wait for the next signal.