Your circuit is probably fine. It's the simulation that needs to be tweaked. The potientimeter has (by default) 100 steps from end to end which gives you 200mV per step. If you double-click the pot, you can change the step-increment to something finer to get better control. Another way to do it is to change the +/-10V power supplies to +/- 1V to further increase the resoultion. You have to remember that the simulation of an analog circuit is still being done in the digital realm. The pot will always have a finite number of steps. You just need to compensate for it in the simulation.
I changed the supply voltages to +/1 1V and the pot increment to 0.1% and was able to adjust the output voltage to 21.201 uV. In the real world, your pot would be more linear and the DMM resolution would (probably) show 0V.
