Yeah, Heather's right.
When a weight loss goal is too aggressive, the numbers end up well below the minimum so it can take quite a lot of exercise input before the range becomes responsive.
I agree that at your weight setting 1 pound a week would be more appropriate. You should have no trouble changing that - as Heather said, Spark doesn't stop you. Not sure why you think you couldn't if you tried, there should be no issue.
If you still can't - why? What happens? What did you do, and what did you get?
Deb, in New Zealand