There are many reasons why strategic plans fail, especially:

Failure to understand the customer

Why do they buy
Is there a real need for the product
inadequate or incorrect marketing research

Inability to predict environmental reaction
What will competitors do
Fighting brands
Price wars
Will government intervene

Over-estimation of resource competence
Can the staff, equipment, and processes handle the new strategy
Failure to develop new employee and management skills

Failure to coordinate
Reporting and control relationships not adequate
Organizational structure not flexible enough

Failure to obtain senior management commitment
Failure to get management involved right from the start
Failure to obtain sufficient company resources to accomplish task

Failure to obtain employee commitment
New strategy not well explained to employees
No incentives given to workers to embrace the new strategy

Under-estimation of time requirements
No critical path analysis done

Failure to follow the plan
No follow through after initial planning
No tracking of progress against plan
No consequences for above

Failure to manage change
Inadequate understanding of the internal resistance to change
Lack of vision on the relationships between processes, technology and organization

Poor communications
Insufficient information sharing among stakeholders
Exclusion of stakeholders and delegates