Update bug?

I have the following statement a bunch of times in a row:

Curvefit......
Coefficients[0,3][rr]=W_coef[p]
Curvefit...
Coefficients[4,7][rr]=W_coef[p]
etc.....

Only the 1st statement is executed correctly, all subsequent statements incorrectly transfer the same 4 values which are W_coef[0], instead of W_Coef[0:3]

W_Coef = nan between statements does not help

Is this a bug or am I doing something wrong?

Thanks!