Wierd Shaping Tweens Happening

I usually don’t get bothered with this problem much but now I feel that I need to know how to fix this problem before making a project and suddenly it getting messed due to this problem.

The problem is that sometimes when I shape tween, the image morphs and distorts into an unimagineable shape.

For example, when I try to shape tweena square that stretches in an angle. It morphs and distorts instead of it actually doing what I want it to do. Lines break and twist and bend and the colours fade and disappear etc.

What is wrong? Like my friends say, Flash is so sensitive.