CKOUT at a low voltage level

On my Arduino Uno R3, I changed the low fuse setting from 0xFF to 0xB7 and saw a 16 MHz output ranging from 0V-5V on pin 9, which is what I wanted. I wanted to see the effect of changing the setting to 0xBF, and saw a 16 MHz signal with a range of 0V-250mV (I don’t remember exactly, but it was quite low). As I understand it, that’s expected. But when I tried to change it back to 0xB7, the voltage level stays low. I’ve set it back to 0xFF and the signal goes away, so I know I am still able to change the fuses, but I can’t figure out why I can’t get the high voltage level. Does anyone know why I am having this trouble?

Thank you.