Problem with neutral type delayed differential equation

Hi!

I am trying to solve a neutral type differential equation with the DDENSD function, however it does not seem to work properly for me. I get results, but no matter how I change the coefficients of the delayed term, the results does not change at all. I attached my code:

function sol = Simulation(Tend)

sol = ddensd(@ddefun,@dely,@delyp,@history,[0.11 Tend]);

figure(1)

plot(sol.y(1,:),sol.y(3,:));

xlabel('u1(t) [m]','fontsize',40);

ylabel('u2(t) [m/s]','fontsize',40);

set(gca,'FontSize',30);

figure(2)

plot(sol.x,sol.y(1,:))

xlabel('t [s]','fontsize',40);

ylabel('u(t) [m]','fontsize',40);

set(gca,'FontSize',30);

figure(3)

plot(sol.x,sol.y(3,:))

xlabel('t [s]','fontsize',40);

ylabel('du(t) [m/s]','fontsize',40);

set(gca,'FontSize',30);

end

%-----------------------------------------------------------------------

function yp = ddefun(t,y,ydel,ypdel)

omega1=2*pi;omega2=2*pi;xi=0.1;r=1;mu=1000000;K=0.01;

yp = [y(2)

-2*xi*r*omega2*(y(2)-y(4))-omega2^2*r*(y(1)-y(3))-mu*r*(y(1)-y(3))^3-omega1^2*y(1)+K*omega1^2*ypdel(4)

y(4)

-2*xi*omega2*(y(4)-y(2))-omega2^2*(y(3)-y(1))-mu*(y(3)-y(1))^3];

end

%-----------------------------------------------------------------------

function dy=dely(t,y)

dy=[0,0,0,0];

end

%-----------------------------------------------------------------------

function dyp=delyp(t,y)

dyp=[0,0,0,t-1];

end

function y=history(t)

y=[0;0;0;0.1];

end

Thank you!