Functional test script stops running suddenly.

My functional Test Complete script has 7 steps. In each step it goes to different step of the application and fills the values. But when it suddenly stops and closes the browser and i have to stop the execution of script. But when i execute that step seperately, it runs fine without the browser getting closed.



I don't understand why this is happening. In the log it says That particular field it comes out says object doesn't exist. 



Can you give me some suggestions for this.