optimization of simulink model using optimization toolbox

We are trying to optimise parameters(x,k) of a simulink model by using optimization toolbox(Lsqcurvefit solver).The optimization is not properly carried out as the one of the parameters('x') always takes the lower bound value,where 0<=x<=4 and 20<=k<=50 Manual procedure for optimisation gives the value for x as 0.2 and k as 43.6.please find attached the simulink model. xdata=[2.63 3.88 5.89 9.06 12.52 15.46 17.84 19.2 19.52 19.6 19.37 19 18.6 18.07 17.24 16.34 15.12 13.71 12.06 10.36 8.44 6.65 5.18 3.88 2.92 2.29 2.12] ydata=[2.41 2.89 3.99 5.81 8.21 10.76 13.31 15.26 16.74 17.75 18.35 18.69 18.8 18.69 18.41 17.98 17.27 16.42 15.29 13.82 12.18 10.33 8.5 6.6 5.04 3.74 2.83]