how to stop fmincon

I am currently using fmincon (I have used it in the past) and am experiencing a problem. I am currently testing a problem and I start with the solution x such that f(x)=0 is minimized (I am minimizing the sum of squares). However, the "line search steplength" is equal to 1 at every iteration, and fmincon does not stop even though f(x) is on the order of 10e-17. I have to manually stop it or set MaxIter to a small number so it stops on its own.

First, is there any reason why this would happen?

Second, Is there anyway to terminate when the function value f(x) is then than some threshold instead of relying on TolX and TolFun conditions?

Thanks.