Regarding usage of ODE15S and FMINCON in parameter estimation

I have a parameter estimation problem as follows:

dx1dt = f(k1,k2,k3, x1,x2,x3) dx2dt = f(k1,k3,k4, x2, x3) dx3dt = f(k1,k2,k5, x1,x2,x3)

all functions nonlinear. i am using ode15s (considering it to be stiff) and fmincon for finding k1, k2, k3, k4 & k5. I have only 9 data points. My query is that when i run the code each time i get different k's. I mean some time my feval is high indicating bad fit and sometimes it is low. why does this happen, why can't the solution be same always.