I have an LPC11U37 design. Some boards will enter ISP mode and mount as a USB drive on the Windows XP box and some will not. I took three of the failing units and verified that each processor has 3.3V power, each has the ISP signal low on power up, each has USB VBUS high (4.7V). The USB connector and solder joints look good for all units. Two of the units have a working clock oscillator and one does not. I've rejected that one as a different problem. What can keep some parts from mounting as a USB device to accept a new image?