Delay differential equation in MATLAB (dde23 )

Question

I've been trying to create a biological model with weekly drug injections that causes few of the immune cells types to react only 7-14 days .The model consists of a for loop with index of the number of weeks, and lags array for the delays. However, if I change delay of 7 days to 14 days I get the same solution. Is there any problem with my code? would really appreciate if you have any idea how to fix it.

%paramaters and delay
a=6;
b=10;
c=9;
lags = [7 ];

y0 = [ 200 9.5*5*10^3 0.5*5*10^3 10^3 100 100];
tspan =[0,6/24];
n_weeks =6;
opt = odeset('AbsTol',1e-9,'RelTol',1e-6);

%Modeling 3 injections
for i=1:3
  
sol =dde23(@(t,y,Z)ddefun(t,y,Z,a,b,c),lags,y0,tspan,opt);
semilogy(sol.x,sol.y(1,:),'r',sol.x,sol.y(2,:),'k',sol.x,sol.y(3,:),'b',sol.x,sol.y(4,:),'m',sol.x,sol.y(5,:),'g',sol.x,sol.y(6,:),'y')
legend('1','2','3','4','5','6')
xlabel('Time t');
ylabel('Solution y');
hold on

y0=sol.y(:,end);
tspan = [7*(i-1)+6/24,7*(i-1)+22/24];
m1=0;
sol =dde23(@(t,y,Z)ddefun(t,y,Z,a,b,c),lags,y0,tspan,opt);
semilogy(sol.x,sol.y(1,:),'r',sol.x,sol.y(2,:),'k',sol.x,sol.y(3,:),'b',sol.x,sol.y(4,:),'m',sol.x,sol.y(5,:),'g',sol.x,sol.y(6,:),'y')
legend('1','2','3','4','5','6')
xlabel('Time t');
ylabel('Solution y');
hold on

y0=sol.y(:,end);
mu1=0;
m1=0;
tspan = [7*(i-1)+22/24,7*i];
sol =dde23(@(t,y,Z)ddefun(t,y,Z,a,b,c),lags,y0,tspan,opt);
semilogy(sol.x,sol.y(1,:),'r',sol.x,sol.y(2,:),'k',sol.x,sol.y(3,:),'b',sol.x,sol.y(4,:),'m',sol.x,sol.y(5,:),'g',sol.x,sol.y(6,:),'y')
legend('1','2','3','4','5','6')
xlabel('Time t');
ylabel('Solution y');
hold on

y0=sol.y(:,end);
tspan = [7*i,7*i+6/24];
end


hold off
%-------------------------------------------
function dydt = ddefun(t,y,Z,a,b,c) %equation being solved
 ylag1 = Z(:,1);

 dydt = [ a*y(1);
b*ylag1(2)*y(1);
c*y(1)+a*ylag1(3);%
y(4);
ylag1(5)*ylag1(6);
y(6)];%

end