Some port pins on PIC's are an open-drain output, and need a pull-up resistor to "make" a high signal. RA4 on smaller PIC's in particular.

This one has tripped me up before also.

I havent used the '870, but you can check the "I/O pin diagrams" in the data sheet to determine which pins are "push/pull" (don't need a pull-up) and which are open-drain (need a pull-up).