Are you sure that you are not trying to source to much out of the output
pin?  I sometimes only use the lower 2 pins on port A, because I try
to make port A always RS232 pins, and I have no problems at all.  
Try to put a pull-up resistor on the output pin and see if it works.
Check to make sure you are not trying to pulse the lines too fast, once
I tried to generate a 2 mhz clock from an output pin until I very
carefully figured out the capacitance in my 22 awg hook-up wire that was
7 inches long. :)

Darryl Palmer