TMinuit doesn't find minimum

I am trying to fitting multiple graphs which share common parameters simultaneously,
and I am finding two problems:

  1. Minuit is struggling to find the correct minimum
  2. when it does find the correct minimum the errors are very large.

I have attached an example piece of code which displays the first problem this is a modification of the tutorial combinedFit.C. The actual fit I am trying to perform is of similar types of curves but is much more complicated in terms of how the common parameters are related. I realise that there is a simple solution to this example and that would be to either change the initial parameters or reduce the noise by increasing nd. But I would like the fit to be more robust against these issues.

if someone could give me any suggestions as to how to get this to work that would be much appreciated.

thanks for your help

combinedFit.C (4.94 KB)