Reasons why a Project can fail:

Below are some of the reasons why projects frequently fail.
Lack of communication. Communication is both the most important responsibility of the project manager and the biggest reason for project failures. It’s the most critical piece of the project management puzzle and it’s something that many of us struggle to do well. As a project manager, you must be ready to be the focal point of communication for your project and carry that task out well.
Poor project planning. If not enough time is spent up front in planning the project and getting a good schedule and the proper documents in place as well as mapping out the resource usage and the budget, then the project can get into trouble quickly. Plan well up front and you set a positive and productive course for your project for the rest of its life cycle. And remember, it will never be cheaper and you’ll never have more time to do the right project planning later on in the project. Do it up front or your project may be doomed before it is even started.
Poor requirements. This is definitely linked to project planning. Whether your project team is helping the customer with all of the requirements definition or if the customer has come to you with detailed requirements, they still need to be reviewed in great detail because missed requirements or poorly documented requirements end up costing the project budget infinitely more dollars down the road in re-work than it requires to just verify and drill down to more detailed requirements up front on the project. Do it right the first time and you’ll greatly lessen the risk of having a project that gets halted when funds run out or the customer is just too frustrated to move on.
Weak leadership. Weak project leadership – meaning a project manager who can’t run a project well – is another major contributor to project failures. The project manager must be a great communicator, a strong leader, an organized project professional, and have the dedication and stubbornness to make good decisions and stick to them. If too many of these characteristics are lacking, the project may flounder or completely fail.
Lessons Learned
And finally, one thing we can do on any project – whether it’s a success or a failure or falls somewhere in between – is to conduct a lessons learned session with the project team and the customer when the project has completed. It’s hard to face the problems when a project goes horribly wrong – especially if you, as the project manager, were somewhat to blame. But the best way to ensure we don’t repeat the same failures is to make sure that we learn from them. So conducting a lessons learned session is definitely the right way to go. And yes, even conduct them on successful projects because there is always something we could have done better.