I too wondered what the purpose of the 3.3/5 volt switch was. After reviewing the schematic, it is only to change the voltage applied to the AVCC pin of the processor, which is used to set the upper range reference. I tested mine and I do get 1023 counts if I apply 3.3 volts to the analog input pin with the switch in the 3.3 volt position. Of course when the switch is in the 5V position, 1023 counts requires 5 volts to the analog input pin.
So what counts are you getting? Have you measured your 3.3 volt pin to see what it’s actual voltage is?
Lefty