If you want something done, ask. Finding fault can prevent both sides from seeking another way to solve a problem. People who don't cooperate have different priorities. To make theirs the same as yours, show how you both will benefit by completing the task.
Some people resent following orders, so be subtle.
Let it go. If you find yourself nagging others too much, it may be time to evaluate your own duties. Recognize the time and effort colleagues put into the process.