Which one.
Many Arduinos with different processors and logic voltages are confusingly called "Nano".
If you mean a classic 5volt/16Mhz Nano, then show us your code/diagram, how you power it, and a picture of the setup. Also tell us why you use an external Aref.
Don't forget that the Aref in has an internal resistance of 32k, which makes a voltage divider with the reference supply resistor.
Leo..