Hi there,
It sounds like you may have your functions/statements in the wrong place. Double check that you have if statement (eg. if field1 is not blank) as the condition and as the value you put the field that you want to write out your value to (eg. field2 = field1).
If that doesn't work for you, perhaps you can include a little more information such as the field names and your condition and I can help debug your problem.