I think that there are two different ways of answering this question.

First, I believe that if we’re talking about failure, it is not necessarily because of the plan. If a business was fundamentally not going to work, even the most thought out and detailed plan in the world would not save it.  Think of all the new startups and apps, backed by investors, that do not work out. I would think that in order for them to get funding from investors they would have needed very thorough business plans, but a good business plan does not guarantee that a venture will be successful.

An example of this would be the app Circa that failed in 2015. Circa was basically an app that aggregated news stories from around the world, but employed a team to read important news stories and then rewrite the important points in a concise format, rather than just posting links to different news websites like google news does. According to an article by the Verge (linked below) Circa was founded in 2011 and raised more than 5 million investor funding. Although I didn’t look through its business plan, I would imagine a company that receives this much funding has a pretty thorough plan. Despite this, the company failed because fundamentally, its concept proved to not work out in the real world. Reasons for its failure can be read in the article below.

http://www.theverge.com/2015/6/24/8842009/why-circa-failed

Secondly, I think that a business plan can fail even for a company that has a fundamentally workable and profitable idea. Reasons are numerous, a lack of preparing for contingencies, lack of research, poor execution, poor management, lack of funding, poor advertising or market exposure, ineffective sales tactics, the list can go on. I believe in this stage it would be important to be flexible and revisit or entirely rework a business plan. I can speak from personal experience on this matter. I am currently working to build a financial planning practice, and am actively growing my client base. Over the summer my natural market ran dry and I was running into difficulties acquiring new clients. A solution that worked for me was to find a new company to work with, and more importantly, find a new mentor. With his coaching I have been able to expand into new markets by changing my marketing and prospecting tactics. In this example, my original business plan was not working and I did not know how to fix it, so I sought out an expert for advice and changed the way I was working. I think the same line of thinking applies to many companies. Perhaps many business owners have a specific vision for how they see things going, and it possibly that their ego or pride can blind them from seeking help.