One other thing, you didn't mention if you have the EN (enable) pin connected to anything. Just something else to check.
OK, one suggestion...as a troubleshooting aid, might I suggest diconnecting the PIC from the bridge and driving the bridge inputs with direct voltage signals from your supply. Flip flop them with a switch or something. I realize you tried the resistor per the link but I would take it one step further. One of the best troubleshooting techniques is to "divide and conquer". If the bridge drives the motor properly with a manual test like this then you have no alternative but to suspect the PIC is not working properly with the bridge chip.