Difference between PostMessage on windows 7 and windows 8

I developed application that use PostMessage to simulate mouse click in another app. I have screenshots of second app, i have coordinates for mouse click and i am simulate mouse click for second app. That all work fine on windows 7, but when i run it on windows 8 i am so confused: screenshots of second app on win8 and win7 are the same, but when i emulate click by PostMessage, click occurred in different area. And i have two questions: 1) Why PostMessage work different on win8? 2) How it can be fixed?