Many project managers wait too long to take action when schedule slippage occurs. Sometimes it's due to inertia, sometimes work overload or bigger fish to fry and sometimes, it's hoping that the problem will fix itself or that "we'll make it up later". This last scenario is probably the most dangerous because it's based on deluding oneself.
While every project is different and the appropriate threshhold at which one should intervene or at least investigate is context-dependent, the fact is that the longer intervention is delayed, the lower the chances of recovery (assuming an achievable schedule to begin with). Remember, schedules slip an hour at a time but those tiny, incremental slips can create a negative trajectory that if not corrected will soon become irreparable.
At a minimum, you need to analyze the variance to determine what the root cause is and collaboratively perform this analysis with the individual responsible for the activity in question. This will allow you to determine if it's a delay due to a faulty initial estimate, due to a flaw in process or approach, etc., and what the impact (if any) will be on the project completion date. Only when you know WHY the variance occurred, can you intelligently figure out WHAT to do about it and HOW to do it in order to get the project back on track. Perhaps the situation is recoverable, perhaps not, but this is a perfect example of a situation where information is power. Whatever the reason for the variance against plan, delay in acting is almost NEVER a good idea. Remember, hope is NOT a strategy.
http://blog.projectconnections.com/alan_koch/2009/11/fix-it-fast-vs-fix-it-right.html - A deliberate Root Cause Analysis process will help you get to the bottom of a problem and fix it permanently, not just quickly.
http://blog.projectconnections.com/project_practitioners/2010/11/three-steps-to-getting-unstuck-lessons-learned-from-a-ground-squirrel.html - Three steps to getting unstuck when facing a challenge or a problem.
http://blog.projectconnections.com/project_practitioners/2009/01/dealing-with-cognitive-dissonance----cognitive-dissonance-is-an-uncomfortable-feeling-caused-by-holding-two-contradictory-ide.html - By doing and committing to solid planning, admitting and being transparent when problems arise, and focusing on a solution for getting back on track you can improve your team’s performance and credibility.
http://www.projectconnections.com/templates/detail/impact-analysis-template.html - When considering a project change or a particular solution, or choosing between multiple possible solutions, it helps to have a thorough understanding of how each option will impact business areas, systems, and external organizations. Will it add to the development time? Will it require additional training, or reduce turnaround times? An Impact Analysis will help you correlate all those variables and determine the best course of action.
http://www.projectconnections.com/templates/detail/problem-solving-tools-techniques.html - Six different problem-solving tools help you do everything from determining root causes to assessing possible solutions, and outline a basic problem-solving strategy so you can be sure you've covered all the bases.