Is is late because of...
Design Changes – Design changes during project execution almost always cause delays and impacts to your budget. Once the Scope document has been signed, any changes to the design need to go through your Scope Change Request Process.
Skill Sets – When planning, assumptions are made regarding people's skills. Sometimes these assumptions turn out to be wrong. Also, you will usually have people on your team who are new or are less experienced. These new or lower skilled workers won't be as productive or effective as higher skilled workers. Make sure your project plan has accounted for skill levels.
Unplanned Work or Workarounds – Many times changes must be made to the sequence of planned work. These changes can impact time, cost, budget, and quality. Think about these risks up front and discuss what if any workarounds will be used.
Rework – Rework happens; it is part of project management. Ensure your project plan accounts for rework.
Team Morale – Turnover, project conflict, sick time, vacations all can wreak havoc with your schedule and budget; plan for these things. A happy team is a productive team. Ensure your team is working towards a common goal and not working against each other. Remove disruptive team members from your project if their behavior can't be changed.
Schedules – Trying to do too much in too little time will result in delays. Once you get behind it is very difficult to catch up. Your project will have delays. You need to have contingency plans to get back on track quickly.
Work Environment – Ensure that your team has a proper workspace. Cramming people into poorly designed work spaces will lower productivity.
Tools – Ensure your team has the right tools to do the job. Having the right tool, but not getting into the teams hands at the right time will cause delays in your schedule.
Project Manager Overload – Too many people on a project team without the proper management oversight can cause major problems for the project manager.
Overtime – Adding hours to people's schedules in order to make a deadline will usually do nothing but increase your budget. Adding overtime rarely results in getting a late project back on track.
Executive – Executive apathy can kill your project. People are usually not going to make your project a priority if their boss isn't willing to tell them it is important. Deadbeat executives kill team morale and project momentum.
Plan for the above "risks" and you will start to bring your projects in faster, cheaper, and with higher quality.