Cut a polygon

I am trying to use the cut command to cut a Polygon into 2. Sometimes it works
and sometimes it doesn't. I would love to know why it works sometimes and not
others. It's obviously I am doing something wrong and when they cut it's just
luck. 

 I am doing the following steps.

1. I select the object
2. I select the polygon.
3. I select the cut button and click in the split check box
4. The cursor changes shape and I drag a line across the polygon.
5. I hit the right mouse button to exit the cut command.

Any idea's of where I'm going wrong. Any help would be great.