Even when very useful process attributes are identified in best practices, such approaches do not by themselves guarantee success for a given situation. This is because:

• Best practices, like prescriptions in medicine, can be over-used and under-used. The trick is in learning when to use them, how much of them to use when they're needed, and how to tell that they're being used correctly.
• Changing the way a group does something requires motivation, a commitment of attention and focus, and implementation resources which often are in higher demand for managing crises and putting out fires than for implementing improvements. In the long term, the best practice is probably the best thing to do. In the short term, it may be more difficult to implement such a change than to deal with the devil you know (assuming you do indeed know it!)
• There are dependencies across practices that may not be immediately apparent. For example, if you want to institute a measurement practice, but haven't put in place the disciplines to manage change, your measurements may not give you the insight you need due to the variation introduced by uncontrolled changes.
• The disciplines and skills required for effecting improvements (especially for activities related to planning and communications of change) are often more demanding than for other types of work. This is because 'changes' are typically held to a higher standard before people 'buy in' to them, since such changes ask people to move away from things that have given them rewards in the past. If the culture and environment is not prepared for moving to a best practice, or you don't have the infrastructure in place to facilitate the change, you should not attempt it prematurely.
• The current approach may be highly dependent upon selected tools or relationships that would be 'broken' with a move to a 'best practice'. You have to consider the readiness of the entire system to move to a best practice, not just the area that is most affected by a change. Practices can't just be 'plugged in', they have to be integrated.