Surface Fitting Custom Equation

I have 2 independent variables, x and y, and data for the dependent variable, z, that I want to fit a custom equation to. I type the custom equation into the curve fitting tool.

a*exp(b*x)+c*exp(d*y)

And the fitting tool varies the coefficients a and c until it outputs a fit. But it doesn't vary coefficients b and d at all from their starting values, which is a big problem. If I manually change the starting points b and d I can get much better fits, but I want the tool to find me the best fit by changing all the coefficients, does anyone know why this isn't working?

[Christiaan]

Dear Daniel,

A manual how to use the Curve Fitting Tool from Mathworks for Surface Fitting can be found here.

If you load your x,y and z data here, you can use 'Custom Equation' for a fit. There you can use the equation: a*exp(-b*x) + c*exp(d*y). Please mind the minus sign in the first exponent. (a plus sign probably wouldn't convergate) Could you send me your data (as attachment), so that I can have a look at it?

Kind regards, Christiaan