You could diversify and still not get the goal you want, diversification is not always the answer to increased profit, when you are very focused on diversification, you could make wrong investment choices in a bid to diversify.
It is only an inexperienced person that would be a victim of diversification. One of the ancient philosophers said, "man know yourself". It includes knowing your strengths and weaknesses. Before embarking on the journey of diversification, it means, sitting down, counting the cost, doing your research, having plan A, B, C... I believe that those who do not achieve their goal of diversification are those who "didn't know themselves". It is possible that they diversified too early and some didn't even need to diversify at all. Anyways, I hope those who noticed their diversification mistakes are humble enough to go back and focus on one investments until they feel ready enough to explore other arears.