Sometimes, no matter how well you try to plan, things just don't work out the way you expected. In some cases, there was something wrong with the assumptions or projections that were made when you first created the plan. In that case, the correct response is to revise the plan to better reflect your actual experience. When you correct your assumptions or projections, you can establish new, more realistic goals for your business.
Unfortunately, sometimes the plan is sound, and the failure to meet targets is a result of problems in your business operations. There can be any number of reasons for this, and your plan can help you figure out what's not right. When you create the plan, you'll try to consider all the factors that might help or hurt your business. By doing so, you'll already have a list of potential problem areas. You can review the plan and try to identify exactly where your operations are deviating from what you wanted. Did you overestimate the demand for your product or service? Are sales people not working out the way you had hoped? Are there problems with back office functions, such as billing? In this capacity, a business plan makes your troubleshooting efforts easier.