Specific ADC input not working on Microchip PIC PIC24FJ128GA202, other inputs work fine
I had the same problem..

PIC24FJ128GA204 Family Silicon Errata and Data Sheet Clarification

AN9 has been removed from all 28-Pin
diagrams and tables. This analog channel is
unimplemented in 28-pin devices.

But datasheet is not repaired..