I wonder if you are fitting data with a user-defined function without auto initialization of the parameters. If so, you might need to initialize the parameters manually. To do that, choose Action: Fit in the dialog box of the advanced fitting tool, and fill in the blanks after the name of each parameter with proper values.
If you are not using a user-defined function, or the above solution cannot help, please tell us again. We will try to solve this problem as soon as possible. ^_^