Can you tell us what current you are passing through your shunt resistor?
The Uno has a 10 bit A/D so each bit is 5V/1024 =~ 5 mv.
With a 75mv/50 amp shunt, 5mv is approx 3.5 Amps.
You can lower the range of the A/D by specifying the internal ref ~ 1.1 volts
this will make each step of the A/D ~ 1mv so you will not be able to detect < 0.7Amps
When I have to troubleshoot an issue like this, I output the RAW A/D converter value.