Not able to fit data points

[Wile_E_Coyote]

Draw your function with your initial values of parameters and see if it behaves like your data.

Fitting data

In ROOT, before you try to fit your graph or histogram, you MUST set “reasonable” initial values for ALL parameters of your function (except for some “built-in” formulas, like “expo”, 1-D and 2-D “landau”, 1-D and 2-D “gaus”, “chebyshev”, “pol”, for which the standard fit procedure can automatically “guess” them). Otherwise, the fitting procedure may easily misbehave.