Sounds like you may have connected to the wrong pin? Note that ADC1 is on PIN8 and ADC2 on PIN7 and so on. So ADC5 is on PIN 4.
If you have connected correctly, did you measure the voltage on the ADC5 pin?
Also can you check your ADC reference voltage. Is it at 5V. If not, check the AVcc Cal potentiometer to get 5V.