There are many reasons why projects (both simple and complex) fail; the number of reasons can be infinite. However, if we apply the 80/20 rule the most common reasons for failure can be found in the following list:

Poorly managed	Undefined objectives and goals	Lack of management commitment
Lack of a solid project plan	Lack of user input	Lack of organisational support
Centralised proactive management initiatives to combat project risk	Enterprise management of budget resources	Provides universal templates and documentation
Poorly defined roles and responsibilities	Inadequate or vague requirements	Stakeholder conflict
Team weaknesses	Unrealistic timeframes and tasks	Competing priorities
Poor communication	Insufficient resources (funding and personnel)	Business politics
Overruns of schedule and cost	Estimates for cost and schedule are erroneous	Lack of prioritisation and project portfolio management
Scope creep	No change control process	Meeting end user expectations
Ignoring project warning signs	Inadequate testing processes	Bad decisions