fminsearch with 3 variables problem: one variable does not change during optimization.

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Hello,

I am fitting a 3-element Windkessel model to some blood flow data (analog circuit shown below). The flow Q and pressure P are given, and I need to find Windkessel parameters R1, R2, and C.

My code minimizes the error

between the fitted data

and original data P using fminsearch. One of my variables, R1, does not change at all while two other variables are far away from the given answer. Does anyone know what is wrong with the code? Thank you very much!

% Global variables pressure, time, Q, p0, dqdt were initialized from raw data
% R1, R2, and C were estimated and not shown here.
theta = [R1 R2 C]; % R1 never changed before and after fitting.
[y,~] = fminsearch(@opt3_4W,theta);
function R = opt3_4W(theta)
    global pressure time Q p0 dqdt 
    % Q is flow; p0 is initial pressure; dqdt is the first derivative of flow over time.
    [t,p] = ode45(@(t,p) ode_3E_4W(t,p,time,Q,theta, dqdt) ,time,p0);
    R = sum(abs((pressure-p).^2));
end
function dpdt = ode_3E_4W(t,p,time,flow,theta,dqdt)
    q = interp1(time,flow,t);
    dq = interp1(time,dqdt,t);
    R1 = theta(1);
    R2 = theta(2);
    C = theta(3);
    dpdt = q/C + (q*R1)/(R2*C) + R1*dq - p/(C*R2);
end