A problem of using analogRead()

I want to use Arduino Mega to control actuators by feeding back the voltages of the actuators. I have successfully used analogRead() to read the voltages from PortF, the analog pin 0 to 7. But I can’t use analogRead() to read voltage at PortK, the analog pin 8 to 15. I read the same invalid voltages at these pins. Do I need to modify any registers of Portk or others?

What version of Arduino are you using? I believe this was a common problem in 0017, but was (I think) fixed in 0018. There's a workaround somewhere here as well..

Thank you, TchnclFl.
I was using 0017, and I updated to 0018 yesterday.
The problem is fixed in 0018. My program works out well now.