There are lots of ways that your project can get off schedule:
Your team might not be as experienced as you expected, so it is taking longer to do the work than you estimated. Also, you might not have allowed sufficient time for team members to become familiar with a task before starting to work on it.
A task required more work effort than you estimated. For example, you might not have identified steps necessary to complete the activity.
A team member might have spent less time on the task than he or she agreed to spend.
Team members are inadvertently expanding the scope of a task.
A particular team member might be more or less productive than you assumed when you developed the plan.
Team members might be reporting the ...