Thank you for the suggestion. I have the pin connected to 15 because:
Pin 16 is for 10 volts
and Pin16 is for 5 volts.
Just noticed this in the datasheet, pg11.
Positive supply voltage must be 1.25V above the desired
reference voltage.
So it looks like you need to use VREF2.5 (jumper 4 & 14).