The most effective project managers know that no project is ever flawless--but they know how to adapt and respond to challenges in order to stay on track.

Big projects rarely finish on time or on budget. The bigger the project, the more it may seem to miss the mark. But if you know the reasons why this happens, you can begin to take steps to avoid these pitfalls when planning your next project.

As a business consultant as well as an Agile-certified coach and trainer, I've seen many project management approaches, from traditional methods to newer ones. I know what works and what doesn't. And while each project has its own challenges, there are a few truths that are absolute for any large undertaking.

And once you know those truths, you can better map out your project's path to success. Here are 7 reasons why a project might go over budget or take too long to complete.

1. It's too complex

Too often than not, the complexity turns out to be greater than the team originally thought.

The bigger the project, the more chances for something to go wrong. There are so many moving pieces in large projects, and you can never assume each piece will come together flawlessly. Things will go wrong. It happens. But a good project manager needs to anticipate that there will be challenges and factor them into their timeline and budget.

2. There's a lack of vision

Nothing will sink a project faster than having no idea what the outcome of it looks like.

If you're going to undertake a big project, you should have a pretty good idea of what the end result should look like. If you say that you want to improve your website's user experience but have no idea what "improve" means, then you don't have a reliable target to determine whether or not you hit your goal. This goes for the whole project as well as the individual groups and employees working on it.

The more clearly defined the project is, the more each component will be able to work toward their individual goals.

3. The "domino effect" throws everything off

Another problem with big projects is that each piece can have a big effect on the others.

If one part experiences a failure, you can bet that it will have an impact on the other parts. A hiccup in one department might throw off the timeline of another department. This is especially true of IT projects or projects that use the Waterfall method--where one piece cannot start until the one before it is finished. This approach becomes especially damaging when problems arise at the end that require fixes back in one of the earlier phases.

4. You don't have an approach for dealing with the unknown

You can't always control what roadblocks you'll run into. You may find challenges in your project you had no idea could arise. You will need to be able to respond effectively to these unknown factors. And you need to be flexible enough to adapt and overcome these situations.

An Agile method can help when faced with many unknowns because you can empirically see what works and pivot when needed. You need an approach for dealing with unforeseen obstacles. It allows you to be more flexible and adapt while not buckling under the pressure of a new threat.

5. Ineffective leadership

Having an absent leader is a sure-fire way to ensure that your project will fall short.

An ineffective leader may be even more damaging for team morale. What is your culture like? Do you listen to your people? I can't even count how many times I went in to consult for an organization and the leaders seem to think things are just fine while the team members are struggling, morale is down, and the people feel like no one is listening to them. 

Servant leadership is an approach that shares power, puts the needs of people first, and helps people develop and perform as highly as possible. Leadership support is key to set the tone for the entire organization.

6. Communication is lacking

Communication is your best tool for keeping projects on track.

All of the above reasons revolve around communication in some way. You need to communicate the goal of the project, the unknowns that arise, the directions of your leaders, or the effects of a failure. Without communication, neither groups, nor individuals will be able to stay aligned with the goals of the project.

Communication assures everyone is on the same page. And when you're on the same page, you can respond better to complications.

7. You've bitten off more than you can chew 

One problem with big projects is that the testing phase usually doesn't come until the end--and fixes can sometimes require a complete reworking.

By only focusing on one small piece at a time, you're allowing your team to focus on what is immediate and attainable. And therefore you can make better predictions of your timeline and budget. Being able to respond to tests from the get-go means any changes that need to take place by the end of the project won't be so drastic, costly, or time-consuming.

It's almost impossible to give a perfect estimate of the time and budget of a large project. There are just too many parts involved, and too many external factors that are difficult to control. The goal then isn't to learn how to give more accurate estimations, but to learn how best to approach big projects in order to make sure they're as effective and efficient as possible.

By knowing the challenges you'll face, you'll be better prepared to avoid their trappings and keep your project out of the quicksand. Leveraging an iterative and incremental approach allows you to release to market ahead of schedule with essential features within the budget, and then build upon that foundation with future releases.