Need help on a SPECIFIC CASE of finding local extreme

I wrote a code to find local extremes and tested it with a few functions. Things were going smoothly until this ONE CASE: f(x)=(x+1)^(2/3)-x. Matlab returns only one local extreme where there should have been two. I also included the ezplot function in my code to see how the graphs look but when Matlab sketch this function's graph it.....just CAN'T, it only graphs the funtion from -1 while the domain of the function itself is much larger.

Note: My code runs with other functions like discontinuos function: abs(x) and ofcourse smoothly with continous functions.

disp('This is MaxImin');

syms x ;

disp(' please input f');

f = input('f(x)=');

g = diff(f);

h = diff(g)

CP = solve(diff(f),x)

s = numel(CP)

for i=1:s

if subs(h,CP(i))>0

disp('Local minima is achieved')

Min=[CP(i);subs(f,CP(i))]

elseif subs(h,CP(i))<0

disp('Local Maxima is achieved')

Max=[CP(i);subs(f,CP(i))]

end

end

ezplot(f);

hold on;

plot(CP,subs(f,CP(i)),'r o');

grid on;