I am trying to build a Biased Random Walk can someone tell me what my mistake is?

I'm trying to build a model for a biased RW, that the probability of the walk, changes depending on the current value, with a lower limit of -0.35. For some reason the probability of the RW does not change when it hits -0.35 and as a result the RW acts like a regular biased Walk with a probability of 0.6. Any help would be appreciated.

%Biased model for Ibm 
N=502;
M=1000;
D=zeros(M,N);
D_sq=zeros(M,N);
AvgD_sq=zeros(1,N);
AvgD=zeros(1,N);
for j=1:M
p=rand(1,M);
for i=2:N
if  D(j,i)>=0 
   prob=0.6;
elseif (D(j,i)>=(-0.2)) && (D(j,i)<0)
   prob=0.7;
elseif (D(j,i)<(-0.2)) && (D(j,i)>=(-0.35))
   prob=0.5;
else    prob=0.2;  
end

if p(j)>prob
   D(j,i)=D(j,i-1)+.01;
else  D(j,i)=D(j,i-1)-.01;

end
D_sq(j,i)=D_sq(j,i-1)+D(j,i)^2;
end
end
AvgD_sq=mean(D_sq);
AvgD=mean(D);
%Graph showing the X^2
figure
plot (AvgD_sq);
set(gca,'FontSize',20);
title ('X^2 prices, Vs. t','FontSize',20);
xlabel ('Time(sec)','FontSize',20);
ylabel ('X^2','FontSize',20);
figure
plot (AvgD);
set(gca,'FontSize',20);
title ('D, Vs. t','FontSize',20);
xlabel ('Time(sec)','FontSize',20);
ylabel ('D','FontSize',20);

Answer 1

[the cyclist]

I did not dig deeply into your code, but I know that every time you get to the line

if  D(j,i)>=0

D(j,i) is always equal to zero, so prob(j,i) will always be equal to 0.6.