Curve fitting

My custom equation does not fit my data properly, and as such gives me a poor R- Square value

I have a challenge using the curve fitter in MATLAB. I want to determine 3 unknown variables using this method. These variables are viscoelastic properties; stiffness, viscosity and relaxatio time. Therefore I am using the 3-paramter genearalized Maxwell model, with formular:

From my analysis, and from the plot, it seems MATLAB is fitting only the linear curve, but isnt taking into consideration the exponential curve.

It is intersting to find that, when I use a different data from a different experiment, I get the parabolic fit with a very high R-square value, which makes me wonder, if it could bfrom my data, or the model itself