SOR Method not running correctly

Hey guys I included my code below as well as a screenshot of the problem I am supposed to be doing. Why is this coming out as a flat surface?

% HW 8

dx = .01; dy = .01;
x = 0:dx:5; y = 0:dy:4; 
imax = length(x); jmax = length(y);

%D=2; a = .3; b = .4; c = 1;
%alpha = D*(2/dx^2+2/dy^2)-c;  

S = @(x,y) -800*(exp(-10*(((x-1)^2) + ((y-2)^2))));

u = zeros(imax,jmax);

error = 0;
omega = 1.95;
itermax = 4000;
tol = 100;
iter = 1;
D=2; a = .3; b = .4; c = 1;
alpha = D*(2/dx^2+2/dy^2)-c;  

H = @(y) 50 + 2*y;
Q = @(x) 50 - 3*x;

while (iter < itermax && error > tol)

      error = 0;

      z = @(x) g(x);
          z = 50 + x;
          if (1 <= x && x <= 3)
              z = 100;
          end

      F(x) =  @(y) 50 -y;

      u(1,1) = (1/2)*(F(1) + g(1));

          for i = 2:jmax - 1
              u(i,1) = g(i);
          end

      u(imax,1) = (1/2)*(g(imax) + H(1));

      for j = 2:jmax - 1
          u(1,jmax) = F(jmax);
          for i = 2:imax - 1
              T = D*((u(i-1,j)+u(i+1,j))/dx^2+(u(i,j-1)+u(i,j+1))/dy^2) ...
                  + a*(u(i+1,j)-u(i-1,j))/(2*dx) + b*(u(i,j+1)-u(i,j-1))/(2*dy) ...
                  + S(x(i),y(j));
              R = T/alpha - u(i,j);
              error = error + abs(R);
              u(i,j) = u(i,j) + omega*R;
          end
          u(imax,jmax) = H(jmax);
      end

      iter = iter + 1;

end

      mesh(x,y,u')