If planning involves working out details for the future, why does it not ensure success?

It is true that planning is a forward looking function and is based on analysing and predicting the future elements. However, future involves uncertainity and cannot be predicted. There are various external forces that affect the functioning of an organisation. These forces are complex and ever changing in nature. For example, social trends, political conditions, technology, government rules and regulations change continuously. Due to such uncertainties, one cannot be sure of the success rate of the plans rather, the plans need to be modified to adapt to the changing environment. For example, suppose due to entry of a competitor, the demand for the products of a company reduces. While planning out production the company could not foresee such a change. Thus, the previous plans of the company fail and it must formulate new plans keeping in view the change in demand. Similarly, suppose the government announces a relaxation in the rules for foreign investment, as a result of which competition in the market increases. The sales and manufacturing plans of the domestic companies might fail in the face of the increased competition. Thus, it can be said that despite the fact that planning involves working out details for the future, it does not ensure success.