mV difference not detected by LM741 opamp comparator
There's an error in observing the valid electrical parameter for Vin CM range (common-mode) on the datasheet.

The input transitors need this to bias  the NPN differential input amplifier.

You need a ground sensing comparator like the quad-LM339  (PNP inputs) or a negative Vee supply of at least 2V on the 741.