PIC24FJ128GA204 - RP6 won't work as output (mikeaparker)

I have a design which uses the 24FJ128GA204 in the QFN 44 package. The design uses RP6 (Pin42) as the SPI3CLKout connecting to the 23LCV512 RAM chip. The problem is that no clock signal appears on this pin. I can waggle the pin with latch writes and I can..